Allow job to set image back to runners default

Allow a job to set the image for the job back to using the runner default. Where a global image is set in the .gitlab-ci.yml there does not seem to be a way to set it back to using the runner's default for a specific job. I have one job in my .gitlab-ci.yml which can use literally any runner (since it has no script section and is used just for archiving artifacts) but I can only run it on runners that allow the globally defined image.

This would either require reserving a keyword such as 'default' or 'runner' that could be set into the jobs image, or allowing an empty value (as opposed to an unset value) to revert to using the runner default image.

Currently, since runner selection isn't done using the image name (see gitlab-ce#15348), it also requires that this task has the tags for it not to fail if sent to a runner that doesn't allow the globally defined image.

What I have been doing now to get around this is not specifying a global image in my .gitlab-ci.yml and using templates to share the image configuration for all other jobs however it might still be nice to implement this feature.