Cells: Build instance doesn't work with kilo api, juno compute

Fix proposed to branch: stable/juno
Review: https://review.openstack.org/206792

This impacts the upgrade capabilities of OpenStack, which is critical in my opinion.

I've targeted the patch straight onto stable/juno, not sure if that is the right thing to do.

Also there is another bug when num-instances is greater that 2. For some reason deserialization fails at the other end because the instance doesn't exist in the DB which is expected as it hasn't made it to that part of the code yet. The bug is in the kilo conductor when it's trying to resolve the flavour of the instance in backporting.

cells doesn't support rolling upgrades yet so if you're deploying cells you're supposed to have them all at the same release level, so this is not critical, probably shouldn't even be high severity.

Since cells is considered experimental and is not a typical deployment this doesn't quite meet the criteria for critical.

Also, it's possible that it's not documented anywhere but we have never made guarantees about the backwards compatibility of nova-cells. It's always been assumed that they will be upgraded in lockstep. Having said that, I think there is value in putting the patches up to help others who may run into this while trying to do something that's technically unsupported.

I'll put up a patch to document this more clearly.

For future references: The devref guide mentions the experimental state [1] as well as the operators guide [2].

[1] http://docs.openstack.org/developer/nova/cells.html?highlight=experimental
[2] http://docs.openstack.org/openstack-ops/content/scaling.html#segragation_methods

Related fix proposed to branch: master
Review: https://review.openstack.org/207076

I think this patch is also related to this issue https://review.openstack.org/#/c/203241/

And the bug associated with the above patch https://bugs.launchpad.net/nova/+bug/1475787

Reviewed: https://review.openstack.org/207076
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=c15a639dea37f2403e67ac381321d3364163463e
Submitter: Jenkins
Branch: master

commit c15a639dea37f2403e67ac381321d3364163463e
Author: Matt Riedemann <email address hidden>
Date: Wed Jul 29 07:30:38 2015 -0700

    cells: document upgrade limitations/assumptions

    We log a warning when using cells that it's experimental, and we have
    several locations in the docs that say it's experimental, but we have
    never documented that rolling upgrades are not supported and that cells
    deployments must be upgraded in lockstep. This adds that information to
    the warning that's logged on startup and to the devref doc on upgrades
    so people know that cells is still a special kitten that must be treated
    differently.

    Related-Bug: #1479181

    Change-Id: Idd57f8daa308543a1733092f427e41b1a205b1d1

Change abandoned by Matt Riedemann (<email address hidden>) on branch: stable/juno
Review: https://review.openstack.org/206792
Reason: We've doc'ed this better on master:

https://review.openstack.org/#/c/207076/

Dropping this from stable/juno.