Unify registry+repo behavior between plugins

[kylelemons]

I noticed this when trying to do a bulk change of our repositories to use the upstreaam plugins/kaniko{,-ecr,-gcr} -- they don't behave the same when you pass

settings:
  registry: example.com
  repo: bar

Specifically:

• kaniko will try index.docker.io/bar
• kaniko-ecr and -gcr will try example.com/bar (which is the behavior I would expect)

So, with the current state, our non-ECR / non-GCR images have to repeat the registry in the settings block, and they are inconsistent with the internal one.

This PR allows use of either the "compatible" setting block like

settings:
  registry: example.com
  repo: bar

or the previously required

settings:
  registry: example.com
  repo: example.com/bar

with the same results of trying to push to example.com/bar

This change attempts to retain backward compatibility to avoid breaking anyone.

[kylelemons]

Gentle ping -- I think this would simplify our configurations a lot and make the behavior consistent between plugins

[kylelemons]

Another nudge; what do you think about this change?

[shubham149]

@kylelemons I didn't get a notification for it. Apologies for the delay.

[kylelemons]

Awesome! No worries, I'm sure you're busy and we were getting toward the holiday season. Do you have an idea what the timeline for merge and release will be?

[shubham149]

I have merged the change.