kind-on-c should use the images digest in the task file

[hoegaarden]

Right now the task file references the task image with its tag. This tag is a strangely computed thing. This was done, because we couldn't (or didn't know how) pin a container image via its digest. So we created our own digest-like tag and used that.

We now found how to pin an image by its digest 🎉:

  [...]
image_resource:
  type: registry-image
  source:
    repository: gcr.io/cf-london-servces-k8s/kind-on-c/kind-on-c
  version:
    digest: sha256:7f5bce3de4ca7b749b0d27f510135b977cd374fb3a0a9f4381cefe466451a610
  [...]  

Thus we want to do the following:

• [x] remove the strange digest-like tag generation
• [x] pin the image by digest (instead of by tag) in the task file

Bonus:

• [x] If easily possible, state in the commit message what actually has changed (update of the base image, update of docker image, ...)

[hoegaarden]

Done in https://github.com/pivotal-k8s/kind-on-c/commit/4a1b8c9b78e88859848fa6d837b57ad155c5c965 and earlier commits. The container diff we have in the commit message now should give a better overview of what has changed.