plugin/kubernetes: correctly set NODATA for ns (#1229)

* plugin/kubernetes: Add GetNamespaceByName

A bare or wildcard query for just the namespace should return NODATA,
not NXDOMAIN, otherwise we deny the entirety of the names under the
namespace.

Add test to check for this in pod verified mode.

* Review

More comments and move namespace code to namespace.go

plugin/kubernetes/handler_test.go

@@ -476,3 +476,14 @@ func (APIConnServeTest) GetNodeByName(name string) (*api.Node, error) {
 		},
 	}, nil
 }
+
+func (APIConnServeTest) GetNamespaceByName(name string) (*api.Namespace, error) {
+	if name == "pod-nons" { // hanlder_pod_verified_test.go uses this for non-existent namespace.
+		return &api.Namespace{}, nil
+	}
+	return &api.Namespace{
+		ObjectMeta: meta.ObjectMeta{
+			Name: name,
+		},
+	}, nil
+}