MAR-243: Drop inventory_hostname use from tests where possible:

- Primarily focused on Ansible playbooks and variable files.
- The variable is still used within some Python test fixtures. Test
behaviour should still be correct, with one advantage being that
lookups should be much faster than using the setup module (which
would require running Ansible code on each remote/test machine).

MAR-236: Add auxiliary object class to LDAP directory for optional countryName and friendlyCountryName attributes:

- Makes it easier to specify free-form optional country name in
LDAP-backed address books.
- Object class uses Majic-specific PEN (Private Enterprise Number)
"namespace" - 1.3.6.1.4.1.40926.417.2.1 (40926 is Majic PEN, 417
stands for LDAP, 2 stands for object class, and 1 is incremental
sequence number). The 417 is result of adding together lower-cased
letters LDAP in their decimal value.

MAR-233: Replace hping3 with nmap in connectivity tests for the xmpp_server role:

- Parametrize the connectivity tests instead of running them in a
loop.

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.