ovn-controller-vtep.at: Fix intermittent test failure.
When testing the recreation of 'chassis' table entry by 'ovn-controller-
vtep'. The removal of 'chassis' table entry by the 'ovn-sbctl' could
cause 'Broken pipe' warning in ovsdb-server.log. This is due to the
race between 'ovn-sbctl' exiting and 'ovn-controller-vtep' adding
the chassis back. So, if the 'ovn-sbctl' exits right when the
ovsdb-server tries to send update of readd of the deleted 'chassis',
the sending will fail with 'Broken pipe' error.
Signed-off-by: Alex Wang <alexw@nicira.com> Acked-by: Russell Bryant <rbryant@redhat.com>