Make list of lowpri domains configurable

roles/mail/templates/postfix-out/transport.pcre

 # Send high-volume, high-spam traffic onto the lower-priority
 # smtp transport.
 /(yahoo|hotmail|live)(\.[a-z]{2,3}){1,2}$/ smtp-lowpri:
-/tiscali\.it$/ smtp-lowpri:
+{% for pattern in postfix_low_priority_domains | default([]) %}
+/{{ pattern }}/ smtp-lowpri:
+{% endfor %}