add firewall plugin (no backend) to default cni config

in order for the fall back mechanisms to work in containernetworking-plugins, the firewall plugin must still be called via the cni configuration file.  however, no backend will be specified as we will rely on cni to do the right thing.

Signed-off-by: Brent Baude <bbaude@redhat.com>

cni/87-podman-bridge.conflist

@@ -26,6 +26,9 @@
         "portMappings": true
       }
     },
+    {
+      "type": "firewall"
+    },
     {
       "type": "tuning"
     }