Add option of configuring primary_hostname

[geerlingguy]

The primary_hostname in the Exim config file sometimes needs to be manually configured, because Exim has trouble picking up the actual hostname of the server (even if hostname -f outputs the correct hostname).

See http://www.jeffgeerling.com/blog/2016/how-get-your-servers-emails-through-gmails-spam-filter-exim for a little more background.

It would be nice to allow this to be configured... if for no other reason than it's one less task I have to maintain in my downstream playbooks for a few LEB servers where Exim is picking up the domain without the TLD (e.g. emails delivered from apache@example instead of apache@example.com.