New override_gem method

I am ready to merge which is meant to simplify and streamline multi-repo development.

If you are currently using a to override the classic UI or other pluggable repos you will need to update that file to use the new override_gem helper method. We have already changed the developer setup on ManageIQ/guides to show the new way to set up things (see

Note that this code is also being backported to euwe and darga, so that if you switch branches it should just work.

I will probably merge in a few hours.