adding houston-hook compatability 

(basically, mongo_url needs to be able to come from settings as opposed to an environment variable).

Background:

We have an app like yourapp.meteor.com and we want to host it's houston admin as yourapp-admin.meteor.com  

1) We try to do that by setting the MONGO_URL of yourapp-admin.meteor.com to the MONGO_URL from yourapp.  Unfortunately, as far as I can tell, you can't just set environment variables on a production environment (if you're hosting on meteor.com).
2) So, we stick the correct MONGO_URL in settings and try to set `process.env.MONGO_URL = Meteor.settings.app_mongourl` on Meteor.startup.  That doesn't work either, since in https://github.com/meteor/meteor/blob/devel/packages/mongo-livedata/remote_collection_driver.js, the RemoteConnectionDriver is only set up once and is not reactive.
3) Since we can't fork Meteor, our solution is to re-initialize the remote connection driver in Houston if the appropriate setting is around.  

Open to other ways of getting around this hack - feedback is very welcome.