Being able to set cpu_mode to "custom" is nice but doesn't do us
much good if users then have to resort to nova_nova_conf_overrides
to set cpu_model and/or cpu_model_extra_flags.
Instead, honor nova_cpu_model and nova_cpu_model_extra_flags variable.
Also add a release note saying that for those users who did previously
set these flags via nova_nova_conf_overrides, they should move
those bits to the newly understood variables.
Change-Id: I889e70826c5c631b86305b512b86fba2b89521b4
58bad8e6b1