Wait for instance to be shut off after "nova stop"

We were rebooting the compute node too fast, and the instance was still
marked as ACTIVE, in the powering-off task state which was never
completed.
Revert "qa_crowbarsetup.sh: Don't try to start testvm after reboot on Cloud 5.x"

This reverts commit 53c4f34b0fc3150e9501498344413cfb12d1d4e2.
Merge pull request #158 from vuntz/fix-compute-reboot

Wait for instance to be shut off after "nova stop"