Bucket names cannot contain underscore

Bucket names cannot contain underscore

As per Amazon s3 spec, bucket names cannot contain
underscore.
https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudtrail-s3-bucket-naming-requirements.html

This (along with other naming convention mentioned in the doc) are being
enforced via https://github.com/ceph/ceph/pull/26787

This patch is to correct the bucket name generated in ragweed
tests as well.

Signed-off-by: Soumya Koduri <skoduri@redhat.com>