Define v3 credentials_client as library

Migrate v3 API credentials_client to tempest.lib and add unit
tests for the service client.
Partially implements blueprint consistent-service-method-names

Change-Id: I0ae04d76738fe6f639d90af8c142bdbc6a1fa83f
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 7ceb9cc..9e828f6 100644
--- a/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
+++ b/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
@@ -12,3 +12,4 @@
       * identity_client(v3)
       * roles_client(v3)
       * inherited_roles_client(v3)
+      * credentials_client(v3)