feat(repository): disallow duplicate artifact refs

[lorin]

Enforce that all artifacts in a delivery configuration have a unique reference.

Fixes #761.

If a user attempts to submit a delivery configuration that violates this, the response payload looks like this:

---
message: "Multiple artifacts are using the same reference: {lhochstein/lorintesttitus=my-artifact,\
  \ emburns/spin-titus-demo=my-artifact}. Please ensure each artifact has a unique\
  \ reference."

(As an aside, I don't think keel should return a 500 error on input that fails validation, but that's the current behavior). (My mistake: returning 500 was not the previous behavior on a validation error)

It will also log a similar error message at the ERROR level:

Multiple artifacts are using the same reference: {lhochstein/lorintesttitus=my-artifact, emburns/spin-titus-demo=my-artifact}. Please ensure each artifact has a unique reference.

[lorin]

@luispollo @asher

[lorin]

Rebased against master

[luispollo]

(As an aside, I don't think keel should return a 500 error on input that fails validation, but that's the current behavior).

I agree. In general we've bee using 400 in that case. Take a look at the ExceptionHandler class -- it converts known exception types into the desired status code/response. You might need to add another exception type to the 400 case.

[lorin]

rebase against master

[luispollo]

LGTM! Thanks for working on this and congrats on your first Spinnaker PR! 🎉

[luispollo]

P.S. It would be nice to add a test case to ExceptionHandlerTests to cover the new handling of ValidationException in another PR. Sorry I didn't catch this earlier.