Set gnocchiclient mitaka to stable/2.2
Set gnocchiclient mitaka to stable/2.2

This is so we dont break mitaka due to a recent change introduced
in gnocchi client introducing osc_lib. see
https://review.openstack.org/#/c/343877/

Change-Id: I35c82883ef637c92151621255a40c9213dfa0c0c

redhat-openstack/rdoinfo,javierpena/rdoinfo