mirror-update

This role sets up the ``mirror-update`` host, which does the periodic
sync of upstream mirrors to the AFS volumes.

It is not intended to be a particularly generic or flexible role, as
there is usually only one instance of the mirror-update host (to avoid
conflicting updates).

At this stage, it handles the mirrors that are updated by ``rsync``
only.  It is expected that it will grow to cover mirroring other
volumes that are currently done by the legacy ``openstack.org`` host
and managed by puppet.

**Role Variables**