MAR-150: Use fixtures for X.509 artefacts in the web_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 default_https_tls_certificate and default_https_tls_key parameters mandatory:

- Dropped the defaults from web_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.

MAR-129: Updated web_server role for Ansible 2.5.x and new Molecule:

- Fixed linting errors.
- Include global linting configuration file in the test configuration.
- Moved variables from the test playbook into group variables.
- Updated how hosts are referenced in tests.
- Fixed deprecation warnings.