Seeing a race condition where lib/neutron code tries to
set the MTU on br-ex before it exists.
Thanks to some good grepping by sdague, it appears that the difference
between lib/neutron and lib/neutron-legacy is that the initial bridge
being created is br-int while in lib/neutron the initial bridge
created is br-ex, which means there must be some kind of warm-up that
occurs between the first bridge that is created by ovs-vswitchd and the
second, and the second one created is much faster.
So instead, let's just wait for the bridge to be created successfully.
Change-Id: I271dc8b6ae5487c80d2a22153b3fc45fb247707f
75a6454097