Add gnocchi plugin support for devstack
Add gnocchi plugin support for devstack

This patch enables gnocchi plugin for devstack.

Partiallly-Implements: bp gnocchi-watcher
Closes-Bug: 1662515
Change-Id: I6614ce6999c9681bd6fafc6c85a3755b5ce8e2dd

openstack/watcher,stackforge/watcher,openstack/watcher,stackforge/watcher