[Backup Service][UI] clarify wording on Create Repository dialog

The field on the Create Repository dialog used for specifying one or more buckets is entitled 'Couchbase Bucket'. This implies that Ephemeral Buckets cannot be specified. But in fact, repositories appear to be created successfully with either Couchbase or Ephemeral buckets. So, we might want to change the title of this field. For example, if we changed it to 'Bucket(s)', we could emphasize in the tooltip that either kind of bucket is accepted.