MAR-189: Remove deprecation-related tasks and tests for database_server role:

- These were relevant in some older versions of Debian and needed
during upgrades from versions that still employed passwords for
system logins/tasks.

MAR-151: Switch to using PyMySQL module for database and database_server role:

- Explicitly specify the path to MariaDB unix socket to enforce unix
socket authentication.

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

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Deploy MariaDB client login configuration prior to setting-up
users/passwords for deprecated feature testing to avoid errors when
password gets changed in the middle of a task loop.

MAR-168: Introduce additional machine in database_server tests for testing deprecated features:

- Updated Molecule configuration, defining an additional machine.
- Set-up the new machine with pre-installed MariaDB server instance,
root login with password, and separate Debian system maintenance
user.
- Set-up configuration files for root and Debian system maintenance
user login.
- Run the default set of tests on the deprecated machine group.

MAR-128: Upgraded tests for database_server 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).