MAR-243: Drop inventory_hostname use from tests where possible:

- Primarily focused on Ansible playbooks and variable files.
- The variable is still used within some Python test fixtures. Test
behaviour should still be correct, with one advantage being that
lookups should be much faster than using the setup module (which
would require running Ansible code on each remote/test machine).

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.