New override_gem method


#1

I am ready to merge https://github.com/ManageIQ/manageiq/pull/13418 which is meant to simplify and streamline multi-repo development.

If you are currently using a Gemfile.dev.rb 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 https://github.com/ManageIQ/guides/blob/master/developer_setup.md#clone-some-plugins).

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.