Avoid throwing an error in tier_server.update_bucket_class

[dannyzaken]

Explain the changes

1. If a newly generated tier name already exists, regenerate it instead of failing the entire operation.
2. This could happen if update_bucket_class is called too frequently and new_name is generated in the exact second.
3. The name itself has no significance, except that it is used as a unique DB index key. This is the main reason to keep it unique

Issues: Fixed #xxx / Gap #xxx

1. https://bugzilla.redhat.com/show_bug.cgi?id=2294723

Testing Instructions:

1. perform frequent updates to the a bucketclass or the backingstores that are in its policy

• Doc added/updated
• Tests added