Default auth_key value in cfssl serve config does not propagate to individual profiles

See comment on #566 in reference to [this gist](https://gist.github.com/benwhite-deltas/b58e9eaaddccb1a16d85ee8deba07259) -

> note that in each of the profiles I have had to specify the auth_key value, as the default auth_key does not seem to propagate to the individual profiles, resulting in a "no authentication profile" bad request message when attempting to sign via the authsign api.