[DO NOT MERGE] 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.

This is the initial work to get a single navigation and is by no means the complete work. This relies initially on https://github.com/theforeman/foreman/pull/1071

Further, I would like the approach taken to be review by at least another Katello developer and @abenari and @isratrade.

More updates will follow to get Katello pages showing within the single navigation.