Try additional Rackspace region

There is a new Rackspace region coming online that may be fast enough to
use in the gate. Add a devstack job which is configured to run on nodes
in the new region in the silent queue, so that we can check out
runtimes without really bothering people.

Change-Id: I9b8a92ac92562581dab3818196e0006e2d69e08d
Merge "Try additional Rackspace region"