MAR-181: Drop support for Debian 9 Stretch from the backup_client role:

- Provide more details on use of pexpect+sftp backed for
Duplicity (backend has to be used on Debian 10 Buster as well, not
just Debian 9 Stretch).
- Switch to using IPs from VirtualBox default allowed host-only
network subnets.

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

- Updated tests.
- Refactored one of the test to determine hostname dynamically.
- Update role reference documentation.

MAR-149: Switch to using File.content_string instead of File.content in tests for all roles:

- The .content variant returns contents as bytes, while the
content_string returns a string (which is what is wanted in
practically all cases).

MAR-152: Drop support for Debian 8 Jessie from the backup_client role:

- Simplify the invocation of GnuPG commands (since we don't have to
massage output formats depending on distribution version anymore).

MAR-129: Updated backup_client role tests:

- Update how the hosts are being referenced in tests (for limiting the
test runs).
- Use "state: present" for package presence/installation.
- Break-up long lines in tasks so they are under 160 characters
length.
- Fixed YAML linting issues.

MAR-128: Upgraded tests for backup_client 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).
- Updated backup server SSH deployment to have chroot the users for a
bit easier set-up, and updated relevants playbooks/tests for this
purpose.