Create two new clients under tempest.lib libary

There are two API v2 actions that only admin has the permission to excute,
according to:
http://developer.openstack.org/api-ref-blockstorage-v2.html#listPools
http://developer.openstack.org/api-ref-blockstorage-v2.html#showBackendCapabilities

Currently, those actions reside on BaseVolumesClient class and I think we
should separate them to new files: tempest/lib/services/volume/v2/capabilities_client.py
                                   tempest/lib/services/volume/v2/scheduler_stats_client.py

Partially implements blueprint consistent-service-method-names

Change-Id: I48e5e19331dd636a093e1817e5c2ea31534ae2c3
diff --git a/releasenotes/notes/13.1.0-volume-clients-as-library-309030c7a16e62ab.yaml b/releasenotes/notes/13.1.0-volume-clients-as-library-309030c7a16e62ab.yaml
index 056e199..6babd93 100644
--- a/releasenotes/notes/13.1.0-volume-clients-as-library-309030c7a16e62ab.yaml
+++ b/releasenotes/notes/13.1.0-volume-clients-as-library-309030c7a16e62ab.yaml
@@ -8,3 +8,5 @@
 
     * volumes_client(v1)
     * volumes_client(v2)
+    * capabilities_client(v2)
+    * scheduler_stats_client(v2)