fixes #3790 - Added support for isolated engine to foreman menu.

[abenari]

Isolated engines needs to run the route helpers in the plugin context. Since the menu is common to the plugins and the main app, menu items urls needs to be computed at the correct context.
This patch add option to the 'menu' method.
An isolated engine needs to declare

 :engine => MyPlugin::Engine

for each menu item.
I was experimenting with declaring an engine is isolated instead of repeating the declaration for each menu item but favored the proposed solution because it lets an isolated plugin writer to interact with foremans controllers too.

This pull request replaces PR #1075

[domcleal]

@ehelms @isratrade could you review please? We've got three PRs now for the same bug :)

[ehelms]

Testing my usual scenarios with this.

[ehelms]

From testing confirms that this should handle the issues with declaring a menu from an isolated engine, as well as the issue of rendering the menu when inheriting the Foreman layout. There were a few edge case items related to the structure of the Foreman layout, and more modifications on the Katello side I had to do to get a page to render. That being said, this looks good to me for now and we can address any other issues that may arise as I build out the rest of the Katello pages.

[ehelms]

ACK for me. I'd also like the divider functionality, but prefer'd I will open a separate issue and include the addition there.

[domcleal]

Looks good combined with #1055, though one more small issue: the top left Foreman "brand" logo uses root_path, but this should be main_app.root_path.

[ehelms]

@domcleal good catch, forgot I was also seeing that when using Katello

[domcleal]

Thanks @abenari, merged as 49a9808 for Foreman 1.4.0.