Set required devstack variables for Ironic+grenade

When sideways migrating from the fake nova driver, do not attempt to
exercise the base side of Grenade using Tempest and Javelin.  Also,
bump the number of VMs enrolled in the Ironic inventory to 3 to
facilitate Tempest concurrency.

Change-Id: I0dc1c1576ff3ff53d1de76af35e6a6f0e2738ac3
Merge "Set required devstack variables for Ironic+grenade"