When a subnet is updated, for example, to disable then
re-enable DHCP on it, if there is no metadata port it
will just return without trying to allocate an IP,
leaving DHCP unusable on the subnet. This could happen
if an admin, even accidentally, deletes the DHCP port
on a subnet while DHCP is disabled.
This also makes OVN behave like ML2/OVS, which will
re-create the DHCP port when the enable_dhcp flag is
changed to false and back to true.
Change-Id: I943f2fb4db9dc33dc372f844d6133faff415befe
Closes-bug: #2015377
267efd2984