We don't want to keep a router model object on the API instance because that makes it much easier to accidentally save it twice and cause consistency issues. On the other hand, we don't want to have to load the thing from Riak again if we've already loaded it to look something up and haven't saved it.

I'd like to be able to build an API instance by passing in a model object and having it grab the relevant fields instead of making the caller do that. (Out of scope for this PR -- we need to take a look at all the classes in vumitools.api once develop-frontend has landed and we can get rid of all the cruft.)