Assign os_region_name a default value

Leaving os_region_name is unspecified leads to RegionAmbiguity errors.
Defaulting to RegionOne (DevStacks default) allows Trove working
out-of-the-box with DevStack and, in the more common case of not being
in a DevStack environment, it will fail with a NoServiceEndpoint error.

Change-Id: I772e6698566c4c8e43ec5aebea5ac9362ce01ee7
Closes-Bug: #1361377
Merge "Assign os_region_name a default value"