mw/kubernetes: rewrite parseRequest

Stop looking at the qtype in parseRequest and make k.Namespace a map.
Fallout from this is that pkg/strings as it is not used anymore. Also
add a few helper functions to make unexposed namespaces easier to see in
the code.

Add wildcard tests to the middleware tests.

middleware/kubernetes/reverse.go

@@ -15,6 +15,6 @@ func (k *Kubernetes) Reverse(state request.Request, exact bool, opt middleware.O
 		return nil, nil, nil
 	}
 
-	records := k.getServiceRecordForIP(ip, state.Name())
+	records := k.serviceRecordForIP(ip, state.Name())
 	return records, nil, nil
 }