MAR-191: Drop Debian 10 Buster support for the backup_client role:

- Internal parameter for Python's pexpect package is no longer
required - newer versions of Debian have fully switche to using the
Python 3 version of it for duplicity.
- Fix prepare playbook task to iterate over list of backup users
instead of hard-coding the values.
- Drop the directory set-up task that seems to serve no purpose (some
leftover from previous refactoring most likely.
- Update supported version information in docs and metadata.

MAR-189: Added support for Debian 11 Bullseye to backup_client role:

- Duplicity on Buster and Bullseye uses different Python version (2 vs
3), install the correct Python pexpect package.
- Switch the helper backup server to using Bullseye, and tweak its
SSHD configuration accordingly.

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-108: Updated tests for backup_server role:

- Refactored how the internal parameters for GnuPG binary name and length of
key ID used in Duply configuration is set-up (use defaults/main.yml instead of
set_facts module).
- Added Debian Stretch as supported platform to molecule tests.
- Replaced use of backup_server role for deployment with regular server with SSH
properly set-up.
- Use IP address for backup server instead of hostname for backup_client
configuration.
- Fixed key lookup for known hosts to use public keys (and not private keys).
- Added public keys of server for deployment in client known_hosts file.

MAR-108: Implemented initial boilerplate for backup_client tests:

- Fixed backup_client role handling of encryption keys.
- Fixed backup server URI (had too many forward slashes).
- Added Molecule instance configuration file for backup server, one backup
client for testing mandatory parameters, and one backup client for testing
optional parameters.
- Implemented playbook for setting-up the test instances.
- Added test data (SSH, GnuPG keys).
- Added dummy (stock) Molecule test file.