MAR-233: Add IPv6 connectivity tests for the mail_server role:

- Include an example of IPv6 address in documentation for the
smtp_allow_relay_from role parameter (has to have square brackets to
make it usable with Postfix configuratin).
- Expand the tests and test parameters to ensure IPv6 is properly
covered.
- Reorder the names in /etc/hosts slightly to ensure the ansible_fqdn
and inventory_hostname match-up. Code should be revisited to
probably drop the use of inventory_hostname from both roles and
tests, and instead rely on ansible_fqdn and ansible_hostname.
- Correct the Dovecot SSL DH parameter specification to use
ansible_fqdn for consistency purposes.