Sandboxes now return 503 instead of 400 on incomplete checksum list.
Return 503 for missing checksum in sandbox commit for retry

When cookbook content is stored in an eventually consistent store
such as S3, a missing checksum on sandbox commit should be
retried to allow the store to update. This matches the behavior
of the OHC API.
Merge pull request #49 from opscode/of/OC-7951

Of/oc 7951