In a multi-region environment, each region is being deployed separately.
Cell discovery, however, would sometimes fail due to it picking a region
different than the one being deployed. Most likely, an internal endpoint
for region A will not be visible from region B. Furthermore, it is not
very useful to discover hosts on a region you're not modifying.
This changes the check to only run against nova compute services located
in the region being deployed.
Change-Id: I21eb1164c2f67098b81edbd5cc106472663b92cb
328e14253d