Add support for os_region_name

For environments with multiple endpoints due to regions, tasks such as
cinder volume attach will not work unless os_region_name is set. This
allows the os_region_name to be passed in.

Change-Id: I5cc41385741054281dafb501742a0d7b26099f88
(cherry picked from commit bdc8fe5df475ce36e1befc54e12c167b4115163a)