Update api-ref location

The api documentation is now published on docs.openstack.org instead
of developer.openstack.org. Update all links that are changed to the
new location.

Note that redirects will be set up as well but let's point now to the
new location.

For details, see:
http://lists.openstack.org/pipermail/openstack-discuss/2019-July/007828.html

Change-Id: Ib1a25cc31b6802655f576711dab38ddfcb73bbdb
diff --git a/tempest/lib/services/compute/server_groups_client.py b/tempest/lib/services/compute/server_groups_client.py
index 0d440d5..89ad2d9 100644
--- a/tempest/lib/services/compute/server_groups_client.py
+++ b/tempest/lib/services/compute/server_groups_client.py
@@ -35,7 +35,7 @@
 
         For a full list of available parameters, please refer to the official
         API reference:
-        https://developer.openstack.org/api-ref/compute/#create-server-group
+        https://docs.openstack.org/api-ref/compute/#create-server-group
         """
         post_body = json.dumps({'server_group': kwargs})
         resp, body = self.post('os-server-groups', post_body)