API change: service state nodes

[William Reade]

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]

-------------------------------------------

Objections? Alternatives?

Cheers
William