Configure a host to be backed up

This role setups a host to use ``bup`` for backup to any hosts in the
``backup-server`` group.

A separate ssh key will be generated for root to connect to the backup
server(s) and the host key for the backup servers will be accepted to
the host.

The ``bup`` tool is installed and a cron job is setup to run the
backup periodically.

Note the ``backup-server`` role must run after this to create the user
correctly on the backup server.  This role sets a tuple ``bup_user``
with the username and public key; the ``backup-server`` role uses this
variable for each host in the ``backup`` group to initalise users.

**Role Variables**

.. zuul:rolevar:: bup_username

   The username to connect to the backup server.  If this is left
   undefined, it will be automatically set to ``bup-$(hostname)``