Currently VMs created by a previous DevStack run still exists even
after re-run stack.sh. This leads to a failure in launching a VM after
the second run of stack.sh. We need to check the reason of the failure
by nova compute log and clean up remaining VMs. It is annoying.

IIRC we cleaned up existing VMs. While I failed to identify which commit
changed this behavior, I believe it is worth recovering it.

This commit changes unstack.sh to call cleanup_nova. cleanup_cinder()
already cleans up LVM volumes and some of them may be used by VMs,
so I believe it is reasonable to clean up VMs in unstack.sh.

Change-Id: I9fcbc5105e443037fada1ef6a76a078145964256