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-161: Make the ldap_server_domain parameter in the ldap_server role mandatory:

- Updated the ldap_server role.
- Removed default value for the parameter.
- Updated tests.
- Updated role reference documentation.
- Updated release notes.
- Dropped the .local from the Molecule instance names.

MAR-150: Use fixtures for X.509 artefacts in the ldap_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: Deduplicate TLS private key/certificate tests for LDAP server role:

- Rename the key/certificate files to match the Ansible inventory
name.
- Move the tests into test_default.py.
- Change the key/certificate extensions to be more descriptie.

MAR-162: Make the ldap_server_tls_certificate ldap_server_tls_key parameters mandatory:

- Updated release notes.
- Updated role reference documentation.
- Updated usage instructions to cover set-up of CA hierarchy earlier on
in the process.

MAR-129: Updated ldap_server role for new Molecule and Ansible:

- Fixed syntax warnings.
- Moved test playbook variables into group_vars.
- Optimised the invocation of test playbook.
- Remove the 'ss' utility from test machines in order to properly test
the unix sockets.
- Fixed how the hosts are being referenced in tests.