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-162: Make the https_tls_certificate and https_tls_key parameters mandatory in php_website role:

- Dropped the defaults from php_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-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.