Add #394: Postfix Virtual Transport (#461)

* Add #394: Postfix Virtual Transport
This makes it possible to specify a lmtp config file, by providing
POSTFIX_DAGENT.

Update - Readme with informations about #394

    * Add Variable ENABLE_POSTFIX_VIRTUAL_TRANSPORT (task)
    * Add Variable POSTFIX_DAGENT (section)

Added Unit tests for virtual transport

* Fix syntax error in test/tests.bats

* Fix Unit Test

test/config/dovecot-lmtp/dovecot-sql.conf.ext

@@ -0,0 +1,139 @@
+# This file is commonly accessed via passdb {} or userdb {} section in
+# conf.d/auth-sql.conf.ext
+
+# This file is opened as root, so it should be owned by root and mode 0600.
+#
+# http://wiki2.dovecot.org/AuthDatabase/SQL
+#
+# For the sql passdb module, you'll need a database with a table that
+# contains fields for at least the username and password. If you want to
+# use the user@domain syntax, you might want to have a separate domain
+# field as well.
+#
+# If your users all have the same uig/gid, and have predictable home
+# directories, you can use the static userdb module to generate the home
+# dir based on the username and domain. In this case, you won't need fields
+# for home, uid, or gid in the database.
+#
+# If you prefer to use the sql userdb module, you'll want to add fields
+# for home, uid, and gid. Here is an example table:
+#
+# CREATE TABLE users (
+#     username VARCHAR(128) NOT NULL,
+#     domain VARCHAR(128) NOT NULL,
+#     password VARCHAR(64) NOT NULL,
+#     home VARCHAR(255) NOT NULL,
+#     uid INTEGER NOT NULL,
+#     gid INTEGER NOT NULL,
+#     active CHAR(1) DEFAULT 'Y' NOT NULL
+# );
+
+# Database driver: mysql, pgsql, sqlite
+#driver = 
+
+# Database connection string. This is driver-specific setting.
+#
+# HA / round-robin load-balancing is supported by giving multiple host
+# settings, like: host=sql1.host.org host=sql2.host.org
+#
+# pgsql:
+#   For available options, see the PostgreSQL documention for the
+#   PQconnectdb function of libpq.
+#   Use maxconns=n (default 5) to change how many connections Dovecot can
+#   create to pgsql.
+#
+# mysql:
+#   Basic options emulate PostgreSQL option names:
+#     host, port, user, password, dbname
+#
+#   But also adds some new settings:
+#     client_flags        - See MySQL manual
+#     ssl_ca, ssl_ca_path - Set either one or both to enable SSL
+#     ssl_cert, ssl_key   - For sending client-side certificates to server
+#     ssl_cipher          - Set minimum allowed cipher security (default: HIGH)
+#     option_file         - Read options from the given file instead of
+#                           the default my.cnf location
+#     option_group        - Read options from the given group (default: client)
+# 
+#   You can connect to UNIX sockets by using host: host=/var/run/mysql.sock
+#   Note that currently you can't use spaces in parameters.
+#
+# sqlite:
+#   The path to the database file.
+#
+# Examples:
+#   connect = host=192.168.1.1 dbname=users
+#   connect = host=sql.example.com dbname=virtual user=virtual password=blarg
+#   connect = /etc/dovecot/authdb.sqlite
+#
+#connect =
+
+# Default password scheme.
+#
+# List of supported schemes is in
+# http://wiki2.dovecot.org/Authentication/PasswordSchemes
+#
+#default_pass_scheme = MD5
+
+# passdb query to retrieve the password. It can return fields:
+#   password - The user's password. This field must be returned.
+#   user - user@domain from the database. Needed with case-insensitive lookups.
+#   username and domain - An alternative way to represent the "user" field.
+#
+# The "user" field is often necessary with case-insensitive lookups to avoid
+# e.g. "name" and "nAme" logins creating two different mail directories. If
+# your user and domain names are in separate fields, you can return "username"
+# and "domain" fields instead of "user".
+#
+# The query can also return other fields which have a special meaning, see
+# http://wiki2.dovecot.org/PasswordDatabase/ExtraFields
+#
+# Commonly used available substitutions (see http://wiki2.dovecot.org/Variables
+# for full list):
+#   %u = entire user@domain
+#   %n = user part of user@domain
+#   %d = domain part of user@domain
+# 
+# Note that these can be used only as input to SQL query. If the query outputs
+# any of these substitutions, they're not touched. Otherwise it would be
+# difficult to have eg. usernames containing '%' characters.
+#
+# Example:
+#   password_query = SELECT userid AS user, pw AS password \
+#     FROM users WHERE userid = '%u' AND active = 'Y'
+#
+#password_query = \
+#  SELECT username, domain, password \
+#  FROM users WHERE username = '%n' AND domain = '%d'
+
+# userdb query to retrieve the user information. It can return fields:
+#   uid - System UID (overrides mail_uid setting)
+#   gid - System GID (overrides mail_gid setting)
+#   home - Home directory
+#   mail - Mail location (overrides mail_location setting)
+#
+# None of these are strictly required. If you use a single UID and GID, and
+# home or mail directory fits to a template string, you could use userdb static
+# instead. For a list of all fields that can be returned, see
+# http://wiki2.dovecot.org/UserDatabase/ExtraFields
+#
+# Examples:
+#   user_query = SELECT home, uid, gid FROM users WHERE userid = '%u'
+#   user_query = SELECT dir AS home, user AS uid, group AS gid FROM users where userid = '%u'
+#   user_query = SELECT home, 501 AS uid, 501 AS gid FROM users WHERE userid = '%u'
+#
+#user_query = \
+#  SELECT home, uid, gid \
+#  FROM users WHERE username = '%n' AND domain = '%d'
+
+# If you wish to avoid two SQL lookups (passdb + userdb), you can use
+# userdb prefetch instead of userdb sql in dovecot.conf. In that case you'll
+# also have to return userdb fields in password_query prefixed with "userdb_"
+# string. For example:
+#password_query = \
+#  SELECT userid AS user, password, \
+#    home AS userdb_home, uid AS userdb_uid, gid AS userdb_gid \
+#  FROM users WHERE userid = '%u'
+
+# Query to get a list of all usernames.
+#iterate_query = SELECT username AS user FROM users