Engine: Adding basic Foreman plugin definition.

This adds the Foreman plugin definition to define a single
navigation menu setup when Katello is installed. Overlapping entities
are scoped by naming and an entire Content menu is added for the
unique entities Katello brings.
Engine Plugin: Cleaning up unused code that results from no longer
supporting our own top level menu.
Merge pull request #3529 from ehelms/engine-plugin

Engine: Adding basic Foreman plugin definition.