In patch  it introduced a binding of DB uniq constraint for L3
agent gateway. In some extreme case the DvrFipGatewayPortAgentBinding
is in DB while the gateway port not. The current code path only checks
the binding existence which will pass a "None" port to the following
code path that results an AttributeError. This patch adds a simple check
for that gateway port, if it is not created, new one.
(cherry picked from commit