MicroShift's rebase step creates tight coupling with specific branch

In rebase commands ( https://github.com/openshift/release/blob/master/ci-operator/step-registry/openshift/microshift/rebase/openshift-microshift-rebase-commands.sh ) two scripts are called directly: rebase.sh and create_pr.py.

However, soon I plan to deprecate create_pr.py with new rebase.py which will wrap rebase.sh and will be able to create/update PRs based on rebase.sh's exit code and handle more scenarios.
In current rebase step form we would have to either:
- introduce rebase.py also on release-4.12 which is planned, but I'd like to use main as a testing area first
- add custom logic that would perform different steps if the branch was release-4.12 which already creates an exception

Idea is to move contents of commands.sh to the microshift repository and have rebase job call only that file which will contain branch specific branch and the auto-rebase procedure (at least scripting part of it) will be contained to the microshift repo

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