One of the requirements to achieve assert:supports-rolling-upgrade TC
tag is to provide a "plan that allows operators to roll out new code to
subsets of services, eliminating the need to restart all services on new
code simultaneously".
This commit documents such rolling upgrade procedure in the devref,
which is last outstanding requirement for Cinder.
Change-Id: I562deafcd4bb9f5666cc35ae77a6bf20ab889612
fc0034b385