Add region_name to service_credentials for devstack
Add region_name to service_credentials for devstack

The devstack plugin should configure region_name in
[service_credentials]. Things work because of the defaults but looking
at other services that talk to services (nova->neutron, heat->everyone)
they all set a region name in their configs. It's already supported by
SL but isn't set in the devstack plugin.

Change-Id: Ic572464d1044e5b7ca5a0c007fbe7f7fbf042f15
Closes-bug: #1566515

openstack/searchlight,openstack/searchlight,openstack/searchlight