Setup backup server

This role configures backup server(s) in the ``backup-server`` group
to accept backups from remote hosts.

Note that the ``backup`` role must have run on each host in the
``backup`` group before this role.  That role will create a
``bup_user`` tuple in the hostvars for for each host consisting of the
required username and public key.

Each required user gets a separate home directory in ``/opt/backups``.
Their ``authorized_keys`` file is configured with the public key to
allow the remote host to log in and only run ``bup``.

**Role Variables**