[Feature Request] Finish upgrade as a separate step

[markspolakovs]

Thank you for creating this plugin! It has saved me a lot of time.

One feature I think would be useful is to finish the upgrade as a separate step.

Use Case: deploy the service, run some tests, then finish the upgrade if they pass (and rollback if they fail)

[EslamElHusseiny]

+1

[sicrossley]

I've created a simple "Confirm" step in my forked repo. I'll add "Rollback" too, and then I'll submit a PR. Please have a look and let me know what you think.
https://github.com/sicrossley/rancher-plugin/tree/confirm-upgrade