Add Postfix main.cf template for setting the hostname.

Although the new settings can be manually configured in moonshine.yml,
by default the values are queried using Facter. This will ensure that
the Postfix `myhostname` and `mydestination` settings will be
automatically updated on the next deploy if a server is renamed.
Merge pull request #212 from bryantraywick/200-add-postfix-main.cf-template-for-setting-the-hostname

Add Postfix main.cf template for setting the hostname.