Fix file exists bug when creating network in devstack.

Remove ip addresses before adding an ip address to dev.

Change-Id: I43843bc3cfdf8fe7f8db7a2a46a2b569f36e5607
Closes-Bug: #1672243

devstack/lib/tacker

@@ -388,6 +388,7 @@ function tacker_create_initial_network {
 
     echo "Assign ip address to BR_MGMT"
     sudo ip link set ${BR_MGMT} up
+    sudo ip -4 address flush dev ${BR_MGMT}
     sudo ip address add ${NETWORK_GATEWAY_MGMT_IP} dev ${BR_MGMT}
 }