Fix rails constants warning

This fixes: lib/foreman_discovery/engine.rb:302: warning: already initialized constant ForemanDiscovery::Engine::MANAGER lib/foreman_discovery/engine.rb:271: warning: previous definition of MANAGER was here

@ofedoren do you agree?

I guess? If it were me I would get rid of those constants unless they are used somewhere else. Even though REX plugin does the same thing and it already has app.reloader.to_prepare, so it works, I'd remove them. There is no much point to use constants here...

Oh, good point. They really are "constants" because they're not frozen.