`sctool backup delete` to not require cluster to be currently managed by scylla manager

[karol-kokoszka]

We have the command sctool backup delete that allows to remove the particular snapshot from the backup location.
https://manager.docs.scylladb.com/stable/sctool/backup.html#backup-delete

This command would be useful in situation where the cluster is removed and the end user wants to free the cloud storage space by removing cluster's backups.

It's impossible right now, because sctool backup delete checks if the cluster defined with --cluster-id flag is managed by the current instance of scylla-manager.

If the cluster with given id doesn't exist, then the command fails.

➜  scylla-manager git:(docs_add_agent_config_change_prereq) ✗ ./sctool backup delete -c ea031891-0357-436c-acea-9fb7f1661a90 -T sm_20230710221000UTC -L s3:backuptest-smoke
Error: get resource: load cluster "ea031891-0357-436c-acea-9fb7f1661a90": not found
Trace ID: JCQyzrn4QK6ZRbBZFJvIxg (grep in scylla-manager logs)

This behavior must be changed.

RESULT:
I'm able to remove the snapshot of the cluster that is not managed by scylla-manager already.