Add "trustee_keystone_region_name" optional configuration parameter
that allows Magnum to specify a region when searching for auth_url
in the Identity service.
This parameter is useful for multi-regional OpenStack installations
with different Identity service for every region.
Task: 22990
Story: 2002981
Change-Id: I5dd70ac0fdcbc19761833ccae3f5496c154f0804