[upgrade][neutron] supported release window on rolling upgrade
The neutron team is discussing how many releases we should support in
RPC messages  (to drop downgrade codes in OVO).
This affects rolling upgrade scenarios. Controller nodes are upgrade
in FFU way, but we cannot upgrade compute nodes at once.
This means controller nodes with N+X release need to talk compute
nodes with N release.
As of now, the neutron team is thinking to support LTS to LTS upgrade
scenarios for major distributions and
N->N+4 looks like the longest window.
Rolling upgrade scenarios affect not only neutron but also other
projects like nova,
so I am sending this mail for broader input.
Akihiro Motoki (irc: amotoki)