Avoid race with udev during ovs agent startup

Avoid race with udev during ovs agent startup

After taking down the veth link between the physical bridge and the integration
bridge call udevadm settle to wait for any udev events to be completely
processed by the operating system before recreating the veth pair.

Some distributions (e.g. openSUSE) have udev rules installed by default that
call e.g. ifdown <interface> during the remove event. If that is processed
after the ovs agent already brought up the veth pair again the veth pair's
link will be down after the agent completed startup and networking will be
broken for all VM instances.

Change-Id: I95520ea96a9804c5261a0c994bbca137535cc37c
Closes-Bug: #1218556
(cherry picked from commit 8d88ee7411d43f148b45d0a145fe32a75765a3ac)