Description of upgrade levels configuration options

Configuration option = Default value	Description
[cells]
scheduler = nova.cells.scheduler.CellsScheduler	(StrOpt) Cells scheduler to use
[upgrade_levels]
cells = None	(StrOpt) Set a version cap for messages sent to local cells services
cert = None	(StrOpt) Set a version cap for messages sent to cert services
compute = None	(StrOpt) Set a version cap for messages sent to compute services. If you plan to do a live upgrade from an old version to a newer version, you should set this option to the old version before beginning the live upgrade procedure. Only upgrading to the next version is supported, so you cannot skip a release for the live upgrade procedure.
conductor = None	(StrOpt) Set a version cap for messages sent to conductor services
console = None	(StrOpt) Set a version cap for messages sent to console services
consoleauth = None	(StrOpt) Set a version cap for messages sent to consoleauth services
intercell = None	(StrOpt) Set a version cap for messages sent between cells services
network = None	(StrOpt) Set a version cap for messages sent to network services
scheduler = None	(StrOpt) Set a version cap for messages sent to scheduler services