Add a label to pods created to run user actions (#4826)

Adds `user-action-pod` label to mark action pods exlicitly
diff --git a/core/invoker/src/main/scala/org/apache/openwhisk/core/containerpool/kubernetes/WhiskPodBuilder.scala b/core/invoker/src/main/scala/org/apache/openwhisk/core/containerpool/kubernetes/WhiskPodBuilder.scala
index ca7627a..ec115e1 100644
--- a/core/invoker/src/main/scala/org/apache/openwhisk/core/containerpool/kubernetes/WhiskPodBuilder.scala
+++ b/core/invoker/src/main/scala/org/apache/openwhisk/core/containerpool/kubernetes/WhiskPodBuilder.scala
@@ -56,6 +56,7 @@
       .editOrNewMetadata()
       .withName(name)
       .addToLabels("name", name)
+      .addToLabels("user-action-pod", "true")
       .addToLabels(labels.asJava)
       .endMetadata()