RestClient: Don't hard code volume service name.

Updates the common/rest_client keystone_auth function so that
it doesn't hard code the 'service_name' for the volume service.

The motivation here is to allow Tempest to run in more environments
where the volume service name may not always be 'cinder'.

Change-Id: I3b892781929642fd42a3dd3b4ba8d44486bfe275
1 file changed