Define volume hosts_client as library

Volume hosts_clients have good interfaces now,
then this patch makes them as a library.

Partially implements blueprint consistent-service-method-names

Change-Id: I964df19bf6b6cf657479aa9e42949e83da3535ae
diff --git a/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml b/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml
index df7e46d..b1fe74a 100644
--- a/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml
+++ b/releasenotes/notes/volume-clients-as-library-9a3444dd63c134b3.yaml
@@ -10,3 +10,5 @@
       * availability_zone_client(v2)
       * extensions_client(v1)
       * extensions_client(v2)
+      * hosts_client(v1)
+      * hosts_client(v2)