Add support for grenade-dsvm-rolling test

When DEVSTACK_GATE_GRENADE_ROLLING=1 is passed in, set
NOVA_COMPUTE_ROLLING_UPGRADE=True in the localrc for grenade to read.
This upgrades everything except nova-compute to make sure we support the
rolling upgrade scenario.

Change-Id: I273b82fe2bb449d497f565fe1e2e14e4ea0a59e9
Merge "Add support for grenade-dsvm-rolling test"