Add keep-backport-label in cherry_pick script

[ChrsMark]

What does this PR do?

This PR adds a keep-backport-label in cherry_pick script. If this flag is set then the label needs_backport of the original PR will be preserved.

Why is it important?

In many cases we want to backport to more than one branches so removing the label by default for any backporting action may remove label even if the backports are not completed yet.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

I tested this manually with #15535 and it works.

@jsoriano @kvch what do you think about this?

[kvch]

I like the idea of letting users decide if the label should be removed or not. However, it would be better if the default behaviour would not change. Meaning the label would be removed by default, unless e.g. --keep-backport-label is passed. Also, do you mind using - instead of _ in the flag name?

[kvch]

One more thing: please update the PR description with the changes.