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.