feature(kiln validate): ensure a release source config exists for each release lock

[crhntr]

This is to fix an issue that occurred when we removed a removed release source.

kiln validate will now fail when a release source for a release specified in the Kilnfile.lock is not found in the Kilnfile.

./Kilnfile.lock

releases:
  - name: banana
    version: 1.2.3
    release_source: missing-release-source

./Kilnfile

release_sources:
  - type: s3
    # id: missing-release-source

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.