MAR-152: Fix idempotence issues with web_server role:

- Reload configuration of PHP-FPM to ensure changes are picked-up.
- Install base packages for PHP applications prior to creating the
directory containing unix socket files (so the package installation
would not change them back).

MAR-132: Added support for Debian 9 (Stretch) to web_server role:

- Introduced internal parameters for controlling differing package
names, service names, and paths for PHP FPM package.
- Added Debian 9 machines to Molecule configuration, including the
client machine.
- Restructured slightly preparaiton playbook to support both Jessie
and Stretch.
- Added custom pytest fixture for having a better way to determine
expected package names etc related to PHP.
- Created copy of private key/certificate pair used for testing of
mandatory parameters (to be used with Stretch machine).
- Fixed invalid specification for hosts on top of which the
connectivity test should be run.
- Updated a couple of task names (avoiding to reference PHP 5).
- Updated documentation.

MAR-114: Updated task syntax for web server/application roles:

- Updated web_server, php_website, and wsgi_website roles.
- Added and removed quoting where it makes sense.
- Switched to using expanded syntax (instead of one-liners).
- Updated ordering of arguments in task definitions.