Honor endpoint override from config for volume

I'm guessing we should do this for everyone, but we have volume on
the brain right now. Rackspace is in the weird situation where
they do support v2 but only have v1 in the catalog (wut) So we
need to override the block-storage enpdoint by config. To do that,
we need to actually honor the config setting over here in OSC.

NOTE: We need to systemically overhaul how we're injesting config
over here - because there's too much variation. But we can leave
that for another day.

Story: 2007459
Task: 39137
Change-Id: Ifddf1ddd5abaa768ab18049c09d18bc269f3a4f5

Author: emonty

openstackclient/volume/client.py

@@ -67,11 +67,15 @@ def make_client(instance):
     # Remember interface only if it is set
     kwargs = utils.build_kwargs_dict('endpoint_type', instance.interface)
 
+    endpoint_override = instance.sdk_connection.config.get_endpoint(
+        'block-storage')
+
     client = volume_client(
         session=instance.session,
         extensions=extensions,
         http_log_debug=http_log_debug,
         region_name=instance.region_name,
+        endpoint_override=endpoint_override,
         **kwargs
     )