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-218: Fix test instance preparation for php_website testing:

- Some of the tasks can only be run without errors on the spun up PHP
website instance due to (un)availability of ansible_eth0 variable.

MAR-218: Switch to using fully-qualified collection names for all tasks:

- Ensures there is no ambiguity when invoking a module.

MAR-218: Fix linting errors related to commands doing unconditional changes:

- Tasks run during the prepare stage of test runs do not matter.
- Some of the role tasks are already effectively running only when
change _will_ happen (when it is detected via variable registered by
a previous task).

MAR-218: Fix linting errors for unnamed plays:

- All plays should be named at this point.
- Improve naming of some plays.
- Group some of the common tasks together to speed-up the prepare
step.
- Minor refactoring to make the prepare playbooks more consistent
across the roles.

MAR-181: Drop support for Debian 9 Stretch from php_website role:

- Keep the mail alias step - it is still relevant for Debian Buster.

MAR-150: Use fixtures for X.509 artefacts in the php_website 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-132: Added support for Debian 9 (Stretch) to php_website role:

- Implemented the necessary changes related to differences between PHP
versions and related paths (PHP 5 vs PHP 7).
- Set the shell for application system account explicitly (workaround
for Debian bug 865762 in Stretch).
- Updated Molecule tests to cover Debian 9.
- Updated Molecule test preparation playbook to account for a number
of differences between Jessie and Stretch (mainly related to mailing
functionality).
- Use more specific host groups in tests.
- Renamed a couple of variables in test for sending out mails to make
it clearer what is being looked up as part of regex matching.
- Updated Molecule tests where certain paths depend on what Debian
release they are ran against.
- Split-up Jessie-specific tests into separate file.

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

- Include global linting configuration file in tests.
- Fixed linting issues.
- Fix deprecation warnings from Ansible (for apt module and
state=installed).
- Updated how the hosts are being referenced within tests.

MAR-128: Upgraded tests for php_website role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (eases
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Switched to extracting hostname instead of hard-coding it in a
couple of tests.
- Fixed some linting issues.
- Updated hostname to include Debian version.