API change: service state nodes

[Kapil Thangavelu]

Excerpts from William Reade's message of Thu Dec 15 15:14:23 -0500 2011:
> As part of the machine constraints story, I propose we add a
> "constraints" key to the service state nodes' content. This would
> contain a list of constraint strings because it's convenient and compact
> and reasonably human-readable (ie we only store the keys that are
> actually set at the service level, rather than having an explicit
> fallback/reset/value for every key). So, the node content would look
> something like:
> 
> -------------------------------------------
> charm: cs:oneiric/mysql-42
> constraints: [arch=amd64, cpu=, mem=16G]
> 

That sounds good, but if the only keys are those stored, what is cpu=?

cheers,
Kapil