add warning if devstack run is slow

there is a timeout condition in devstack-gate which seems to be hinted
at by the nodes themselves being really slow. We should emit a warning
when the time to run the devstack setup exceeds 2x the expected time.

Change-Id: If05f949c5785d24572652196f52f56c6cb5b6d64
Merge "add warning if devstack run is slow"