MAR-151: Added support for Debian 10 Buster to web_server role:

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Refactor the code around handling of different directories and files
for PHP 7.0 (Debian Stretch) and PHP 7.3 (Debian Buster).
- Separate socket directory tests for WSGI and PHP applications (due
to differences in paths for PHP in Debian Stretch and Debian
Buster).