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/openvswitch_agent b/lib/neutron_plugins/openvswitch_agent
index 69e38f4..b4a53e4 100644
--- a/lib/neutron_plugins/openvswitch_agent
+++ b/lib/neutron_plugins/openvswitch_agent
@@ -104,7 +104,7 @@
         sudo ovs-vsctl -- --may-exist add-port "br-$VLAN_INTERFACE" $VLAN_INTERFACE
 
         # Create external bridge and add port
-        _neutron_ovs_base_add_bridge $PUBLIC_BRIDGE
+        _neutron_ovs_base_add_public_bridge
         sudo ovs-vsctl -- --may-exist add-port $PUBLIC_BRIDGE $PUBLIC_INTERFACE
 
         # Set bridge mappings to "physnet1:br-$GUEST_INTERFACE_DEFAULT"