don't use screen for grenade

now that the process stop pieces of devstack work much better,
it's not clear that we need to actually use screen to stuff in
the service starts / stops.

We should try to do this without screen.

Change-Id: Icc5b9b5f8dcde22f2864d3ba6aa91d0afe936d7a
Merge "don't use screen for grenade"