MAR-192: Added support for Debian 12 Bookworm to xmpp_server role:

- Some of the tests are still failing, namely the ones centered around
the sendxmpp tool (which seems completely broken at this point in
Debian 12 Bookworm)

MAR-192: Align IP ranges used for testing VMs in the xmpp_sever role:

- Use two consecutive ranges for Debian 11 Bullseye like in
mail_server.

MAR-189: Added support for Debian 11 Bullseye to xmpp_server role:

- Roll-out LDAP client configuration since Bullseye does not come with
a stock one at /etc/ldap/ldap.conf that sets the trust anchor
correctly for validating LDAP server certificates.
- Drop the backports pinning in case of Bullseye (for now let's try to
keep the Buster and Bullseye at same versions for simplicity).
- Drop installation of Python apt bindings (no longer used).
- Tests for Buster and Bullseye need to be split-up a bit due to some
differences around backports etc.

MAR-181: Fix incorrect IP address in tests for xmpp_server role:

- Should match the parameters-optional-buster server.

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

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Drop Stretch-specific workarounds, code, and tests.

MAR-151: Added support for Debian 10 Buster to xmpp_server role:

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Enable lower-level TLS protocols (1.0/1.1) in global OpenSSL
configuration file on Buster in order to be able to test the
xmpp_server_tls_protocol parameter (otherwise Prosody completely
refuses to use them even if listed in its configuration).
- Move stretch-specific tests into its own file (for backported
lua-ldap library), and run them on Debian 9 Stretch machines only.

MAR-150: Use fixtures for X.509 artefacts in the xmpp_server role:

- Removed the statically generated artefacts.
- Generate X.509 artefacts for tests using Gimmecert.
- Updated paths to point to generated artefacts.
- Introduced cleanup playbook for removing generated artefacts.

MAR-132: Added support to xmpp_server role for Debian 9 (Stretch):

- Updated tests to include Debian 9 in testing. Existing private keys
are reused where possible (since most of the naming is identical
between the machines with jessie/stretch).
- Updated invocation of sendxmpp in tests as workaround for
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=854210.
- Updated testing of imported keys to accomodate differences between
gpg/gpg2 (used by apt-key in Jessie/Stretch).

MAR-132: LDAP-related fixes to xmpp_server role tests:

- Do not create LDAP accounts through the ldap_entries parameter of
ldap_server, use the ldap_entry and ldap_attr modules directly.

MAR-129: Updated xmpp_server role for Ansible 2.5.x and new Molecule:

- Include top-level lint configuration in test configuration.
- Fixed linting errors.
- Moved test playbook variables into group_vars and host_vars.
- Fixed deprecation warnings from Ansible.
- Updated how hosts are referenced within tests.