Migrate backups clients to tempest.lib

Split and migrate v1 and v2 backups__client to tempest.lib.

Partially implements blueprint consistent-service-method-names

Change-Id: I9d6ef9f069c449f392c245fdac10996517a1cc73
diff --git a/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml b/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml
index 4886f16..9cfce0d 100644
--- a/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml
+++ b/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml
@@ -6,6 +6,7 @@
     so the other projects can use these modules as stable libraries without
     any maintenance changes.
 
+    * backups_client
     * encryption_types_client (v1)
     * encryption_types_client (v2)
     * qos_clients (v1)