MAR-218: Switch to using task imports instead of includes:

- Should result in somewhat faster run, except the includes happen
during planning phase. None of the changed includes will have
problem with this.
- Solves the issue of (included) imported tasks not being tagged
properly, particularly in relation to the mechanism for explicitly
running all handlers.

MAR-218: Enable case-sensitive column name handling for database privileges:

- This will become a new default in the next major release of the
community.mysql collection.
- Gets rid of the Ansible mysql_user module warning.

MAR-218: Switch to using fully-qualified collection names for all tasks:

- Ensures there is no ambiguity when invoking a module.

MAR-218: Fix include invocations:

- The old include keyword/task has been deprecated in favour of more
explicit include_tasks one.

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-148: Fixing linting errors:

- Do not compare against booleans when value is already boolean in
"when" conditions.
- Use pipefail in conjunction with setting the shell to Bash when
extracting the backup encryption key IDs.
- Do not compare variables to empty strings in "when" conditions. This
change currently works only for complex variables (stuff can break
badly in case of simple variables in current default Ansible
configuration).
- Fix missing space in one of Jinja2 templates.

MAR-135: Switch to using run_handlers instead of handlers as variable:

- Updated all roles to now depend on run_handlers being set to True in
order to force execution of handlers.

MAR-114: Updated task syntax for database roles:

- Updated database and database_server roles.
- Added and removed quoting where it makes sense.
- Switched to using expanded syntax (instead of one-liners).