Takes the existing mechanism for determining Vmdb::Plugins, and moves
the logic into it's own class with a interface that can be repeated with
other Registry providers (working title...)

This can allow other things to provide the plugin registry, like
bundler, or a yaml config instead of relying on Rails to do that for us.

This doesn't remove the concept of using Rails engines to additional
components to our routes and such, but removes it from being a
dependency for defining plugins in the manageiq application.

Uses Bundler as the source for determining vmdb plugins to be loaded.