MAR-28: Implemented backup tests for mail_server role:

- Reworked slightly how /etc/hosts is set-up to allow for multiple names to be
associated with the same IP in test playbook.
- Updated LDAP server to behave as backup server as well.
- Enabled backups on server used for testing optional parameters.
- Implemented backup tests.
- Added necessary test data for backups (keys etc).