add sahara to devstack-gate default runs

sahara is an integrated project, put it in the integrated test
matrix.

Change-Id: I711a7a30460db91e76814088431400df4b9de272
update grenade branches

make it possible to run grenade from stable/icehouse -> master

Change-Id: If46d1624e0b35a37d5dfb043edb513fc04b6ea68