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

- Dropped the enforce_https parameter.
- Updated tests.
- Updated release notes.
- Update role reference documentation.
- Update usage instructions.

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-29: Implemented tests for php_website role:

- Install some additional tools for testing everything.
- Updated test playbook to change allowed extensions for running PHP scripts on
parameters-optional.
- Updated error page to use correct extension for parameters-optional test
instance.
- Expanded rewrite configuration slightly for parameters-optional.
- Install libmariadb-client-lgpl-dev-compat to test mysql_config symlink
creation.
- Deploy a number of PHP pages used for testing if pages are served correctly.
- Set file permissions on deployed PHP FPM pool configuraiton files.
- Use expanded syntax when deploying TLS keys/certificates in order to avoid
issues with TAB mangling.
- Fixed set-up of Strict-Transport-Security header when HTTPS enforcement is
disabled.
- Added a number of PHP and static test pages.
- Wrote tests covering full functionality of the role.