Improve cells in Nova/devstack so that it can be used for a gating check

There are a few issues with running a gate job with cells that need to be addressed.
1. Currently even basic actions like booting an instance fail. Most likely devstack needs some work to configure cells properly.
2. Cells does not support all nova features. Picking a subset of devstack exercises and/or tempest tests will be necessary for a while.
3. Other issues likely to be discovered.