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.