Handling cluster label change for Backup management. Before changing cluster label for a service the code does the following:
1. Removes all scheduled backup tasks for all impacted* services.
2. Checks for running backup/restore tasks for all impacted* services. In case such found, raises error and stops execution. In this case deleted tasks remain deleted (no rollback), labels have old values (label change is not performed).

impacted sevices are all such services that:

• the target one for changing custer label

• services with old cluster label of target service in case label is not empty

• services with new cluster label of target service in case label is not empty

Calling API shall support changing any "standard" label.

For testing the FB for https://jira.percona.com/browse/PMM-11148 can be used.

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