This patch makes the OVN_L3_CREATE_PUBLIC_NETWORK configuration True by
default. This option makes the OVN lib in DevStack create & configure
the external bridge, matching the same behavior from the OVS driver
in DevStack.
Change-Id: Icda53b95fdc3c169ac48a6ec4343c87ba404baa4
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>