split up stop_nova to isolate stop_nova_compute

Split stop_nova into: stop_nova_compute and stop_nova_rest. This is
needed to support the partial-ncpu grenade test where we want to stop
everything but nova_compute.

Patch in master: I6a21821277e56897d705ca5746806e2211632d12

Change-Id: I031707a49974df684cc473737ceb07cecc262c66
Merge "split up stop_nova to isolate stop_nova_compute" into stable/havana