enable smtp tls sending encryption

[tobiasquinn]

This enables postfix smtp tls send which allows it pass the check found at http://www.checktls.com/perl/TestSender.pl

[thegreeny]

Better, there should be a parameter, instead of hard-coding mtp_tls_security_level to encrypt.
A general solution would be setting postconf -e key:value pairs as in PR #40