relates to pull request #372

The previous behaviour relied on managesieve copying over the
.dovecot.sieve file into the user's directory. I found this to be
particularly fragile. For instance, re-deploying roundcube without
dovecot could overwrite the .dovecot.sieve symlink and break
managesieve.

A better approach is to use the global sieve configuration that
roundcube provides and not mess with dovecot's files directly.

Use the changed `mail_virtual_users` structure to ensure directories and
sieve rules are created for each defined account.

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}}"