Post rails4 usability tweaks and fixups

Several useful things in this pull request:

* Make sure that we set v6prefix default value properly.
* Get rid of application_controller methods we no longer use.
* Remove version checking for the API in the barclamp controller.
* Fix barclamps_controller index action.
* Allow the framework to mount other rails engines.
* Make docker-admin smarter about mounting things in /opt/opencrowbar.
* Improve logging of fatal errors.
* Change the default logging level in production to info.
* Make IP address allocation avoid deadlocks harder.
* Allow selecting returned JSON object attributes.