Fix placement_client group help docs generation

To get the placement_client OptGroup help text to generate
in the docs, we have to use the OptGroup object in the
list_opts() method rather than a string for the group name.

https://docs.openstack.org/oslo.config/latest/cli/generator.html#defining-option-discovery-entry-points

Change-Id: Ie728081caa205ded2435c6b95f5e8d4bbd23372c

watcher/conf/placement_client.py

@@ -38,4 +38,4 @@ def register_opts(conf):
 
 
 def list_opts():
-    return [(placement_group.name, placement_opts)]
+    return [(placement_group, placement_opts)]