Ensure RAX's nova-agent is stopped in ironic job

Need to be more diligent when working around the libvirt bug
and ensure Rackspace's nova-agent is really stopped so that
/proc/xen can be unmounted.  The nova-agent init script apparently
does a poor job of this.

Change-Id: Ib906fb035d7820be9514a136b161fd71999a795b
Merge "Ensure RAX's nova-agent is stopped in ironic job"