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.

MAR-196: Simplify allowed TLS protocol configuration for mail_server role:

- Accept minimum version allowed instead of arbitrary list.
- Fixes deprecation warnings in Dovecot logs (ssl_protocols ->
ssl_min_protocol transition).

MAR-181: Drop support for Debian 9 Stretch from mail_server role:

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Use Debian Buster for helper machines.
- Drop Stretch-specific code and tests.

MAR-151: Use 2048-bit DH parameter for IMAP server under Debian 10 Buster:

- Deploy a statically-generated DH parameter.
- Set-up DH parameter configuration based on Debian version.
- Implemented test for newly-generated file.