Gracefully add provider network port

Patch 5ec6f8f introduced the provider network support in DevStack. However, this patch
does not include a port clean up routine during ./unstack that openvswitch complains
that the PUBLIC_INTERFACE already exists and exits when you run DevStack multiple times.
Adding --may-exist to ovs add-port command solves this problem.

Change-Id: I89dc560ffb35fccf6ceed2557047adca37054ce7

lib/neutron

@@ -699,7 +699,7 @@ function start_neutron_agents {
     run_process q-dhcp "python $AGENT_DHCP_BINARY --config-file $NEUTRON_CONF --config-file=$Q_DHCP_CONF_FILE"
 
     if is_provider_network; then
-        sudo ovs-vsctl add-port $OVS_PHYSICAL_BRIDGE $PUBLIC_INTERFACE
+        sudo ovs-vsctl --no-wait -- --may-exist add-port $OVS_PHYSICAL_BRIDGE $PUBLIC_INTERFACE
         sudo ip link set $OVS_PHYSICAL_BRIDGE up
         sudo ip link set br-int up
         sudo ip link set $PUBLIC_INTERFACE up