debian stretch slim (#784)

* Switch to stretch-slim as base image.
 - first step correct the testdata, as newer packages are more strict
about the mail-structure.

* Switch to stretch-slim: correcting the test-environment and the build
 - add missing build-step to make
 - clean the userdb aswell
 - use timeout of netcat, as postgrey would not close the connection
 - there is 2 extra mail-logs -> assert_output 5
 - cosmetic: use "" instead of ''

* Switch to stretch-slim:
new image:
 - smaller size
 - 0 CVEs compared to 11 CVEs in ubuntu 16.04 Image
better backport situation
 - postfix 3.1.6 vs 3.1.0
 - fail2ban 0.9.6 vs 0.9.3
 ...
changes needed because of stretch-slim:
- add missing gnupg and iproute2 package
- remove non-free rar, unrar-free should do
- rsyslog does not add syslog user and has different conf-structure
- pyzor command discover was deprecated and is missing in the new
stretch package

- dovecot does not know SSLv2 anymore. removed because of warnings in
log

- iptables does not know imap3, IMAP working group chose imap2 in favor
of imap3

* Switch to debian stretch slim:
SSLv2 seems to be a not known protocol anymore - good!

* switch to debian stretch slim:
make this test more stable. there might be more than only one mail.log
(mail.info, mail.warn, ...)

* switch to debian stretc slim:
 new openssl 1.1.0 needs stronger ciphers, removed some weekers ones.
Please, look through the new list of cipher! this needs to be done in
another commit for all other SSL/TLS-Endpoints aswell.

* Switch to debian stretch slim:
let our server pre-empt the cipher list.
Did a read through, wwwDOTpostfixDOTorg/FORWARD_SECRECY_READMEDOThtml
and
wwwDOTpostfixDOTorg/TLS_READMEDOThtml

* Switch to debian stretch slim: lets give this openssl-based test a new and independent but identical container.  many other test on the main 'mail' container might interfere here.

* Switch to debian stretch slim: remove unused lines

target/postfix/main.cf

@@ -1,6 +1,6 @@
 # See /usr/share/postfix/main.cf.dist for a commented, more complete version
 
-smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
+smtpd_banner = $myhostname ESMTP $mail_name (Debian)
 biff = no
 append_dot_mydomain = no
 readme_directory = no
@@ -24,10 +24,12 @@ smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
 #smtp_tls_CAfile=
 smtpd_tls_security_level = may
 smtpd_use_tls=yes
+smtpd_tls_loglevel = 1
 smtp_tls_security_level = may
 smtp_tls_loglevel = 1
 tls_ssl_options = NO_COMPRESSION
-tls_high_cipherlist=ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA:!DSS
+tls_high_cipherlist=ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-CCM8:ECDHE-ECDSA-AES256-CCM:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA256:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256
+tls_preempt_cipherlist = yes
 smtpd_tls_protocols=!SSLv2,!SSLv3
 smtp_tls_protocols=!SSLv2,!SSLv3
 smtpd_tls_mandatory_ciphers = high
@@ -79,4 +81,3 @@ policyd-spf_time_limit = 3600
 
 # Remove unwanted headers that reveail our privacy
 smtp_header_checks = pcre:/etc/postfix/maps/sender_header_filter.pcre
-