Automatic merge from submit-queue

AWS/GCE: Spread PetSet volume creation across zones, create GCE volumes in non-master zones

Long term we plan on integrating this into the scheduler, but in the
short term we use the volume name to place it onto a zone.
    
We hash the volume name so we don't bias to the first few zones.
    
If the volume name "looks like" a PetSet volume name (ending with
-<number>) then we use the number as an offset.  In that case we hash
the base name.