MAR-197: Implement environment indicator for the web_server role:

- Now the environment indicator will also cover the default landing
page for the web server.
- Tweaked the landing page to look a bit better in terms of
margins (particularly important for the environment indicator).

MAR-189: Refactored web_server tests to be less hostname-dependent:

- Use the inventory name (alongisde distribution version) in places
where it's possible and makes sense to do so.
- Rearrange the IP allocation a bit to make more sense.
- Drop architecture information from the hostnames.
- Make the connectivity test operate over a matrix of input
parameters.
- Replace singular use of wget for testing with curl.

MAR-170: Always enforce use of HTTPS in the web_server role:

- Dropped the default_enforce_https parameter.
- Updated tests.
- Updated release notes.

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.