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

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Replace the installation of libmariadbclient-dev-compat library with
atftp - the actual package is differently named under Debian Stretch
and Debian Buster (which would complicate the test without any
benefits).
- Drop the fix for root mail alias in Vagrant image - seems it's no
longer a problem.
- Split-up the test for web application user since it's not possible
to keep it all under one parametrised test due to differences in
assigned system UID numbers for 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.