Add image specification to cloud-push

[jubrad]

Allow images such as balancer or orchestratord
to be specified in cloud push. This is useful for cloud personal stacks and self-managed deploys.

Motivation

I want an easier way to push up orchestratord and balancer images. This does have an issue with self-managed where we're not buliding console images so we can't actually push those and the console will break unless we specify an already pushed image in the MZ spec, but it helps test balancer changes a bit.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[def-]

I haven't used this script, so I'm not sure I'm the best reviewer.