Tweak authenticate event definition
Tweak authenticate event definition

The event definition for an authenticate event is simply:
  identity.authenticate

there is no trailing period.

Change-Id: Ie9c3fd66abea2b02411e6f3a01051f59a94bf284
Closes-Bug: 1439538

maestro-hybrid-cloud/ceilometer,pczerkas/aodh,pkilambi/ceilometer,sileht/aodh,chungg/aodh,sileht/aodh,idegtiarov/ceilometer,maestro-hybrid-cloud/ceilometer,cernops/ceilometer,openstack/ceilometer,r-mibu/ceilometer,openstack/aodh,eayunstack/ceilometer,r-mibu/ceilometer,mathslinux/ceilometer,fabian4/ceilometer,isyippee/ceilometer,idegtiarov/ceilometer,redhat-openstack/ceilometer,cernops/ceilometer,isyippee/ceilometer,pczerkas/aodh,fabian4/ceilometer,ityaptin/ceilometer,openstack/aodh,chungg/aodh,redhat-openstack/ceilometer,pkilambi/ceilometer,ityaptin/ceilometer,eayunstack/ceilometer,mathslinux/ceilometer,openstack/ceilometer