Add overlay2 to Docker storage driver dropdown

Change-Id: I382a56ddf4ecf7e76b1a0ecb4446754c1ceecce3
(cherry picked from commit 565614ff35)

magnum_ui/static/dashboard/container-infra/cluster-templates/workflow/workflow.service.js

@@ -350,7 +350,8 @@
                           type: 'select',
                           titleMap: [
                             {value: "devicemapper", name: gettext("Device Mapper")},
-                            {value: "overlay", name: gettext("Overlay")}
+                            {value: "overlay", name: gettext("Overlay")},
+                            {value: "overlay2", name: gettext("Overlay2")}
                           ]
                         }
                       ]