Don't log WARN if S3 bucket prefix does not end with slash

Currently if one configures a S3 bucket prefix without a trailing / in the XML config (or in nuxeo.s3storage.bucket_prefix), Nuxeo emits a WARN log saying

bucket_prefix myprefix S3 bucket prefix should end with '/' : added automatically.

As we're automatically correcting the value anyway, there is no value in having this be a WARN message, and the message is actually harmful when doing analysis because it's not an actual problem.