Don't pass the repository name as the default bucket prefix to lambda picture conversions

Trying to use https://github.com/nuxeo/nuxeo-lambda-pictureview-conversion to delegate the picture conversions to lambda.
By default, the lambda functions is looking to download form the s3 bucket an entry with a key like 'default:$digest' even when we don't use a repository prefix and results in the following error:

Downloading default:36625684a4a96b52093d73624acda34e from: nbm-0-dev-int :
NoSuchKey: The specified key does not exist.

This should be configurable in case we don't use the prefix 'default' with the s3 bucket