In case when port_forwarding service plugin is enabled and vlan or flat
network (provider network types) is configured as one of the
tenant_network_types in the ML2 config there is an issue with
centralized and distributed traffic.
FIP port forwarding in ovn backend are implemented as OVN Load balancers
thus are always centralized but if "enable_distributed_floating_ip" is
set to True, FIPs are distributed. And in such case it won't work as
expected as either it tries to send FIP PF's traffic as distributed when
"reside-on-redirect-chassis" for LRP is set to "false" or
tries to centralized everything (even FIP which should be distributed)
when "reside-on-redirect-chassis" is set to "true".
It's not really easy to avoid that issue from the code so this patch
adds warning in the upgrade checks and also log warning about it during
start of the neutron server process to at least warn cloud admin that
such potential issue may happen in the cloud.
Related-Bug: #2028846
Change-Id: I398f3f676c59dc794cf03320fa45efc7b22fc003