Fix Savanna d-g jobs

DEVSTACK_GATE_SAVANNA env var that was defined in devstack-vm-gate-wrap.sh
should be used instead of DEVSTACK_SAVANNA to enable Savanna in d-g jobs.

Change-Id: I9e4596f277476a9972755a2ca01c12a59e902dcc
Merge "Fix Savanna d-g jobs"