Move volume service clients under tempest.lib

Tempest provides a stable library interface and I think we can use it
for volume clients.

Partially implements blueprint consistent-service-method-names

Change-Id: Ia2d87360a6d88e5f54ecd00efbf6730b06484c99
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
new file mode 100644
index 0000000..056e199
--- /dev/null
+++ b/releasenotes/notes/13.1.0-volume-clients-as-library-309030c7a16e62ab.yaml
@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    Define volume service clients as libraries.
+    The following volume service clients are defined as library interface,
+    so the other projects can use these modules as stable libraries without
+    any maintenance changes.
+
+    * volumes_client(v1)
+    * volumes_client(v2)