Make Swagger API support optional, so that consumers can define their own

OpenShift would like to also enable swagger, but we need to register our
services as swagger services prior to the SwaggerAPI being started. I've
added a bool (default false) to master.Config to enable swagger, and split
the method in master out so that a downstream consumer can call it.

No functional change for Kube, just more master flexibility.
Didn't you want to be able to configure the path at which it's exposed, also?