Introduce PUBLIC_BRIDGE_MTU variable to set br-ex MTU

This variable can be used to accommodate for underlying infrastructure
that does not provide full 1500-sized traffic, or maybe instead gives
access to Jumbo frames.

Change-Id: I38a80bac18673a30842a7b997d0669fed5aff976
Related-Bug: #1603268
diff --git a/lib/neutron_plugins/ovs_base b/lib/neutron_plugins/ovs_base
index ecf252f..9e1421f 100644
--- a/lib/neutron_plugins/ovs_base
+++ b/lib/neutron_plugins/ovs_base
@@ -105,11 +105,16 @@
         sudo ip link set $Q_PUBLIC_VETH_EX up
         sudo ip addr flush dev $Q_PUBLIC_VETH_EX
     else
-        _neutron_ovs_base_add_bridge $PUBLIC_BRIDGE
+        _neutron_ovs_base_add_public_bridge
         sudo ovs-vsctl br-set-external-id $PUBLIC_BRIDGE bridge-id $PUBLIC_BRIDGE
     fi
 }
 
+function _neutron_ovs_base_add_public_bridge {
+    _neutron_ovs_base_add_bridge $PUBLIC_BRIDGE
+    set_mtu $PUBLIC_BRIDGE $PUBLIC_BRIDGE_MTU
+}
+
 function _neutron_ovs_base_configure_nova_vif_driver {
     :
 }