make it Graphite-0.9.12 ready.

- Graphite project has been moved from Launchpad to Github.  Version 0.9.10 source is available at both locations, but 0.9.11 and newer (0.9.12, to the moment) can be found at Github only. It can be easily solved by just overriding attribute `node['graphite']['carbon']['uri']` in the role, but changing a cookbook is better because it avoids unnecessary role complexity.

- Replace all `node['apache']['user']` usage with `node['graphite']['user_account']` in all recipes to use conventional naming.

- Make carbon Upstart scripts 0.9.12 compatible. Not sure if it is Carbon code or underlying Twisted-11.1, but carbon-cache (and carbon-relay as well) Upstart job gets stuck on non-existing PID, which makes job restart impossible. A proposed fix is to run it in foreground by `exec carbon-{cache,relay}.py --debug start` and redirect stdout to console.log (similar to sv-carbon-run way).

If anyone sees this Upstart problem too -  please make a short comment.

Cheers, Anatoliy D.