Respect a requested PortalIP if possible

This will make DNS possible on a static IP.
Yeah, we don't properly version constants today but we should.

----- Original Message -----
> > @@ -552,6 +552,12 @@ const (
> >  	// Status code 202
> >  	StatusReasonWorking StatusReason = "Working"
> >  
> > +	// StatusReasonForbidden means the requested update operation cannot be
> > completed
> 
> I wonder if it makes sense to keep common definitions like this in a
> non-versioned place - it should only ever be accumulative.
> 
> ---
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> https://github.com/GoogleCloudPlatform/kubernetes/pull/1982/files#r19347293