Don't fail horribly if not a state server.
Move the job start to the api server part.
Merge pull request #1220 from howbazaar/fix-system-identity-writer

Don't fail horribly if not a state server.

The agent code still fails, but with a specific error code if it is missing.
The call site is then able to be intelligent.

(Review request: http://reviews.vapour.ws/r/529/)