Teach Grenade to do Juno to Kilo upgrades

First step in doing Juno to Kilo upgrades with Grenade is setting the
new base and target branches. Note this should come after both devstack
and grenade have stable/juno branches.

Change-Id: I0a7c8b3273b25e9fe457d7073046ad8a7ee3f70a
Merge "Teach Grenade to do Juno to Kilo upgrades"