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/conf.d/auth-ldap.conf.ext

@@ -0,0 +1,33 @@
+# Authentication for LDAP users. Included from 10-auth.conf.
+#
+# <doc/wiki/AuthDatabase.LDAP.txt>
+
+passdb {
+  driver = ldap
+
+  # Path for LDAP configuration file, see example-config/dovecot-ldap.conf.ext
+  args = /etc/dovecot/dovecot-ldap.conf.ext
+}
+
+# "prefetch" user database means that the passdb already provided the
+# needed information and there's no need to do a separate userdb lookup.
+# <doc/wiki/UserDatabase.Prefetch.txt>
+#userdb {
+#  driver = prefetch
+#}
+
+userdb {
+  driver = ldap
+  args = /etc/dovecot/dovecot-ldap.conf.ext
+  
+  # Default fields can be used to specify defaults that LDAP may override
+  #default_fields = home=/home/virtual/%u
+}
+
+# If you don't have any user-specific settings, you can avoid the userdb LDAP
+# lookup by using userdb static instead of userdb ldap, for example:
+# <doc/wiki/UserDatabase.Static.txt>
+#userdb {
+  #driver = static
+  #args = uid=vmail gid=vmail home=/var/vmail/%u
+#}