This introduces two new roles for managing the backup-server and hosts
that we wish to back up.

Firstly the "backup" role runs on hosts we wish to backup.  This
generates and configures a separate ssh key for running bup and
installs the appropriate cron job to run the backup daily.

The "backup-server" job runs on the backup server (or, indeed
servers).  It creates users for each backup host, accepts the remote
keys mentioned above and initalises bup.  It is then ready to receive
backups from the remote hosts.

This eliminates a fairly long-standing requirement for manual setup of
the backup server users and keys; this section is removed from the
documentation.

testinfra coverage is added.

Change-Id: I9bf74df351e056791ed817180436617048224d2c