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.
It actually turned out better to be the same.  If necessary we can tweak the swagger config in our own code - I exposed the method for now though.

----- Original Message -----
> > @@ -69,11 +69,15 @@ type Config struct {
> >  	PortalNet             *net.IPNet
> >  	EnableLogsSupport     bool
> >  	EnableUISupport       bool
> > +	EnableSwaggerSupport  bool
> 
> Didn't you want to be able to configure the path at which it's exposed, also?
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/GoogleCloudPlatform/kubernetes/pull/2945/files#r21854923