Add --force to kubectl delete and explain force deletion

--force is required for --grace-period=0. --now is == --grace-period=1.
Improve command help to explain what graceful deletion is and warn about
force deletion.

test/e2e/kubectl.go

@@ -144,7 +144,7 @@ func cleanupKubectlInputs(fileContents string, ns string, selectors ...string) {
 	}
 	// support backward compatibility : file paths or raw json - since we are removing file path
 	// dependencies from this test.
-	framework.RunKubectlOrDieInput(fileContents, "delete", "--grace-period=0", "-f", "-", nsArg)
+	framework.RunKubectlOrDieInput(fileContents, "delete", "--grace-period=0", "--force", "-f", "-", nsArg)
 	framework.AssertCleanup(ns, selectors...)
 }