Define the v3 Users Client as a library

Migrate the v3 API Users Client to tempest.lib and add unit tests
for the client.

Partially implements blueprint consistent-service-method-names

Change-Id: I32441765d1f9028f5ddaaddf045f534181b44ad3
diff --git a/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml b/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
index 4b26c30..6673616 100644
--- a/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
+++ b/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
@@ -7,3 +7,4 @@
 
       * identity_client(v2)
       * groups_client(v3)
+      * users_client(v3)