MAR-31: Implemented tests and small fixes for xmpp_server role:

- Updated test playbook to deploy simple XMPP non-interactive tool.
- Updated test playbook to include an additional administrator for server
hosting domain2 and domain3.
- Updated test playbook to enable backup on parameters-optional test instance.
- Use expanded syntax for deploying TLS key and certificate in order to prevent
mangling of tab characters.
- Set permissions on deployed Prosody configuration file in order to prevent
LDAP password from being world-readable.
- Implemented tests for backup functionality.
- Implemented tests for client connectivity.
- Implemented a number of tests that ensure XMPP server is deployed correctly.
- Full functionality cannot be tested (like MUC and file proxy) due to partial
lack of CLI-based tools.

MAR-31: Added scaffolding for testing xmpp_server role:

- Fixed wrong default for xmpp_domains parameter (should be a list).
- Added Molecule configuration file with LDAP server, client, and two XMPP
server instances.
- Added test playbook that sets-up all servers.
- Fixed mode syntax issues (use lead zero).
- Added data required for testing.
- Added dummy file with tests.