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-171: Make the xmpp_domains parameter mandatory:

- Updated release notes.
- Updated role reference documentation.
- Dropped the default value for parameter from the xmpp_server role.
- Updated tests.

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-162: Make the xmpp_tls_certificate and xmpp_tls_key parameters mandatory in xmpp_server role:

- Dropped the defaults from wsgi_server role.
- Updated group variables in role tests.
- Changed the key/certificate file extensions to be more descriptive.
- Updated role reference documentation.
- Updated usage instructions to include the mandatory parameters.
- Deduplicated tests for the TLS files.

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.