Explicitly require MySQL server as part of OpenDMARC isntall.

Resolves #410.

roles/mailserver/tasks/dmarc.yml

@@ -1,6 +1,7 @@
 - name: Install OpenDMARC milter and related packages
   apt: pkg={{ item }} state=installed update_cache=yes
   with_items:
+      - mysql-server
       - python-mysqldb
       - opendmarc