Add notifications class to provider class group

Add neutron::server::notifications class to provider network class group
so that Nova <-> Neutron notifications get configured.

Change-Id: I5e39a84ca04558e8650199ad88938a51700c869e
Partial-Bug: #1353671
(cherry picked from commit bdcbc81f5e9344dab7b337ddaf458f4b9b8870e2)