[SPARK-12870][SQL] better format bucket id in file name

for normal parquet file without bucket, it's file name ends with a jobUUID which maybe all numbers and mistakeny regarded as bucket id. This PR improves the format of bucket id in file name by using a different seperator, `_`, so that the regex is more robust.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10799 from cloud-fan/fix-bucket.