Cleanup grenade machinery in prep for Juno

Over the Juno cycle we made changes to how we process grenade config in
devstack gate. We did so in a backward compatible way for our old jobs.
Clean up the now no longer needed backward compat code to make new Juno
grenade testing simpler.

Change-Id: I08736a5a1d3d55fe20b95c740e69f4557add46fc
Merge "Cleanup grenade machinery in prep for Juno"