fix(mail): main_virtual_xx variables usage

mail_virtual_xx list were not use correctly in tasks.

We got error 'unicode object' has no attribute 'name' because:
with_items:
 - "{{ mail_virtual_xx }}
is understood has a list of strings containing information extracted from {{ mail_virtual_xx }} variable

 Now we use:
   with_items: virtual_domain_xx}}"
Merge pull request #104 from bcachet/feature/FixMailVirtualListUsage

fix(mail): main_virtual_xx variables usage