Refactor initial service broker registration

* Handle the logic of pinging the broker and saving the record in a
  separate registration model.
* Use a dedicated presenter to serialize the registration instance to
  JSON.
Check the service broker catalog on registration
Present the broker instead of the registration

That is what the user is expecting to create, after all. This obviates
the need for any weird delegation from the registration to the broker,
so as to make the registration quack like a broker to the user.
Inject broker dependency into the registration

Also refactored service_brokers#update to use registration model.
Include query string when making GET requests
Load catalog when registering service broker
remove dead code