Avoid flushing br-ex during stacking

This operation seems vestigial, as it was added to the code when stack.sh
did not have a robust cleanup procedure. These days, unstack.sh does destroy
all bridges, therefore during subsequent stack.sh runs (or even initially, from
a clean environment), the flush operation has become superfluous.

Its removal has also been deemeded necessary to enable certain multi-node
cloud deployments, like the one available in OpenStack infra [1].

[1] https://review.openstack.org/#/c/158525/

Change-Id: I6b4e5b82958e6d29dd450f1c4c9513f6a9e5053a

lib/neutron_plugins/ovs_base

@@ -93,11 +93,8 @@ function _neutron_ovs_base_configure_l3_agent {
         sudo ip link set $Q_PUBLIC_VETH_EX up
         sudo ip addr flush dev $Q_PUBLIC_VETH_EX
     else
-        # --no-wait causes a race condition if $PUBLIC_BRIDGE is not up when ip addr flush is called
         sudo ovs-vsctl -- --may-exist add-br $PUBLIC_BRIDGE
         sudo ovs-vsctl br-set-external-id $PUBLIC_BRIDGE bridge-id $PUBLIC_BRIDGE
-        # ensure no IP is configured on the public bridge
-        sudo ip addr flush dev $PUBLIC_BRIDGE
     fi
 }