set concurrency for grenade

We never tell grenade how parallel to run tests, which actually
mean it's running with all the cores, instead of half the cores.

This paired with: Ieea93dcbb973ca8ff872bf509324db284538d0b5 is
needed to control that.

Change-Id: Ia67345164761fa5810b7593f20a088a006d35001
Merge "set concurrency for grenade"