mitigate the number of times we tag an image

[bacongobbler]

According to @lavalamp, Workflow clusters in GKE are triggering this bug. This issue occurs because Workflow will re-tag an image even if no changes were made. We used to do this because we would inject environment metadata into the image when deis config:set was called. Now, we just inject that environment metadata directly into the k8s manifest, so no image modification (and therefore no image re-tagging) is required.

While the upstream bug will eventually get fixed, in the meantime we should investigate how we trigger this bug, and see if there is a way to mitigate this issue.