Setup backup server

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

Note that the ``borg-backup`` role must have run on each host in the
``borg-backup`` group before this role.  That role will create a
``borg_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 ``borg`` in server mode.

**Role Variables**

.. zuul:rolevar:: borg_retire_users
   :default: []

   A list of backup user names that are in a "retired" state.  The
   host should not be in the inventory or active.  The backup user
   will be diabled and when running a prune, we will only keep the
   latest backup to save space.

.. zuul:rolevar:: borg_purge_users
   default: []

   A list of backup user names whose data should be purged.  This list
   represents backups for hosts that have been retired and we now
   agree we do not want to retain any of their data.