Fix bare variable depreciation

Newer versions of Ansible complain when using bare variables like
`mail_virtual_users` and prefer the newer `'{{ mail_virtual_users }}'`
syntax.

Noticed the depreciation warning with Ansible version 2.1.2.0.

roles/mailserver/tasks/dovecot.yml

 
 - name: Ensure mail domain directories are in place
   file: state=directory path=/decrypted/{{ item.name }} owner=vmail group=dovecot mode=0770
-  with_items: mail_virtual_domains
+  with_items: '{{ mail_virtual_domains }}'
 
 - name: Ensure mail directories are in place
   file: state=directory path=/decrypted/{{ item.domain }}/{{ item.account }} owner=vmail group=dovecot
-  with_items: mail_virtual_users
+  with_items: '{{ mail_virtual_users }}'
 
 - name: Copy dovecot.conf into place
   copy: src=etc_dovecot_dovecot.conf dest=/etc/dovecot/dovecot.conf