Adds/updates notifications test cases
Adds/updates notifications test cases

- Adds a test case showing that the event_callbacks structure can
  contain a single callback for an event that is not in a list.
- Changes the test case that shows how a list of callbacks works to have
  multiple callbacks in the list.

Change-Id: I4b16955694139e8443e01d5a9b726f55efbd88f0

openstack/keystone,ilay09/keystone,dstanek/keystone,rajalokan/keystone,roopali8/keystone,idjaw/keystone,takeshineshiro/keystone,jonnary/keystone,mahak/keystone,mahak/keystone,rajalokan/keystone,maestro-hybrid-cloud/keystone,jonnary/keystone,jamielennox/keystone,roopali8/keystone,dims/keystone,rajalokan/keystone,dstanek/keystone,himanshu-setia/keystone,openstack/keystone,takeshineshiro/keystone,openstack/keystone,himanshu-setia/keystone,maestro-hybrid-cloud/keystone,jamielennox/keystone,dims/keystone,klmitch/keystone,klmitch/keystone,cernops/keystone,ilay09/keystone,dstanek/keystone,mahak/keystone,cernops/keystone,ajayaa/keystone,ajayaa/keystone,idjaw/keystone,ilay09/keystone