Fix S3 bucket property name

The key used to specify a prefix in S3 conflicts with the bucket key in freemarker.

In the S3 package, the bucket to use is specified by nuxeo.s3storage.bucket.
Optionally, a prefix to use for the files in the bucket can be specified by nuxeo.s3storage.bucket.prefix

If the prefix is used, there is the following error on startup:

FreeMarker variables: ignored 'nuxeo.s3storage.bucket' conflicting with 'nuxeo.s3storage.bucket.[prefix]'

The nuxeo.s3storage.bucket.prefix will be replaced by nuxeo.s3storage.bucket_prefix