MAR-108: Implemented tests for the backup_client role:

- Updated test playbook to deploy a simple script for testing pre-backup
handles.
- Added sample known_hosts to test data for comparison purposes.
- Implemented a number of tests covering common set-up, set-up using only
mandatory parameters, and set-up using optional parameters.

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.