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

- Ensures there is no ambiguity when invoking a module.

MAR-218: Quote all octal values in YAML files:

- Fixes linting errors, and ensures there is no ambiguity in case of
YAML specification changes.

MAR-218: Fix include invocations:

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

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

- Updated role reference documentation.
- Updated role meta information.
- Updated tests.
- Do not use distribution version-specific SSH configuration file for
backup server SSH daemon.

MAR-163: Reformat multiple skip-QA directives and remove some unnecessary ones:

- Update codes used to identify the disabled QA checks.
- Switch to using new syntax that disables only a specific QA check
instead of all of them on a single task.
- Drop disabling QA checks related to octal file permissions (this was
most likely bug in older versions of ansible-lint).

MAR-148: Fixing linting errors:

- Do not compare against booleans when value is already boolean in
"when" conditions.
- 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).

MAR-134: Fix Ansible warning about use of apt and with_items:

- The apt module can accept list of names already, and with_items is
considered to be deprecated in this regard (otherwise it would
end-up running one apt command per item).
- Changed multiple roles where apt was used together along with_items.

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

- Deploy slightly different sshd_config file for Stretch (DSA key
unused).
- Updated Molecule tests to cover testing on Debian 9.
- Updated role reference documentation.

MAR-129: Updated backup_server role tests and fixed some issues:

- Updated how the tests are referencing the relevant hosts to run the
tests on.
- Use "state: present" for package installation.

MAR-114: Updated task syntax for backup roles:

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

MAR-107: Minor Ansible linting fixes for backup_server role:

- Use proper mode (with leading zero).
- Deploy the backup server SSH keys via template in order to ensure the files
end with newlines (otherwise OpenSSH server in Debian Jessie might not pick-up
the ed25519 key).