Force means force for enable-app-engine-project.

Reviewed in http://codereview.appspot.com/5798078/.
diff --git a/bin/enable-app-engine-project b/bin/enable-app-engine-project
index e2f2a35..9456d5d 100755
--- a/bin/enable-app-engine-project
+++ b/bin/enable-app-engine-project
@@ -103,7 +103,7 @@
   for dir in argv[1:]:
     # Check if the supplied directory is an App Engine project by looking
     # for an app.yaml
-    if not os.path.isfile(os.path.join(dir, 'app.yaml')):
+    if not FLAGS.force and not os.path.isfile(os.path.join(dir, 'app.yaml')):
       sys.exit('The given directory is not a Google App Engine project: %s' %
                dir)