enable gating on grenade

create a new gate-grenade-devstack-vm-stable for stable trees, as
grenade is not yet working on stable.

turn on voting in check and gate for grenade in master, as we seem
to be only catching legitimate upgrade failures now.

Part of bp:upgrade-testing-in-gate

Change-Id: Ic5776b58d3f660d782111665ae1744574bc5a484