Engine: Initial work to allow Katello to be loaded as a Rails engine
alongside Foreman and be able to access the Foreman application.

Note, this does not enable any UI/API capabilities, the scope of this
initial commit is to get Katello's dependencies loaded and Foreman
running with Katello as an engine mounted.
Merge pull request #3173 from ehelms/engine

Engine: Removing rabl restriction to let Foreman proper handle.
nu-users-engine - work in progress