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.