Services that expose multiple endpoints use a prefix in their keys, this
patch refactors that code to put it in their own function to be reused
by the notifications functionality and make it notificate for changes in
those endpoints (e.g. neutron-api and nova-cloud-controller).
Change-Id: Ieecfc4ef7c85c7f716ceef0c2938ae0c7787953d
Closes-Bug: #1856419