Artifacts retention policy

needs labels: ci and artifacts

I loved the new feature of 8.9 about artifact expiration date (http://docs.gitlab.com/ce/ci/yaml/README.html#artifactsexpire_in) but I think Gitlab needs an extra configuration on artifacts retention policy.

Do you think a last n builds retention policy would be useful?

We could thus have a config like:

job:
  artifacts:
    expire_in: 1 week
    keep_last: 3

To have artifacts expire after 1 week of age but always keep the last 3 builded artifacts.

What do you think?