MAR-191: Drop support for Debian 10 Buster from the php_website role:

- Move some of the distribution release-specific information used in
tests into a fixture of its own. Make the code a bit cleaner/more
consistent.

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-151: Added support for Debian 10 Buster to php_website role:

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Refactor the code to take into account differences in PHP-related
paths between Debian Stretch and Debian Buster.
- Make the test for web application user less dependant on what the
actual UID number is in case of default value. By default user
should be created as system user, which means its UID number should
be less than 1000.
- Drop the installation of libmariadbclient-dev-compat library - the
test is good enough without it, and the actual package is
differently named under Debian Stretch and Debian Buster (which
would complicate the test without any benefits).

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.