allow impersonate serviceaccount in cli

We can impersonate four kinds of resources according to the code:
https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apiserver/pkg/endpoints/filters/impersonation.go#L83

pkg/kubectl/cmd/create_role.go

@@ -77,6 +77,10 @@ var (
 				Group:    "",
 				Resource: "users",
 			},
+			{
+				Group:    "",
+				Resource: "serviceaccounts",
+			},
 			{
 				Group:    "",
 				Resource: "groups",