Support arbitrary subpaths inside S3 bucket

For S3, bucket names can be provided in two ways. Either in trailing form:

http://<S3_HOST_NAME>/<BUCKET_NAME>

or using DNS-style buckets (recommended):

http://<BUCKET_NAME>.<S3_HOST_NAME>

An URL with a subpath can be provided at repository creation, for example:

cvmfs_server mkfs ..... -w http://bucketname.s3.cern.ch/cvmfs

The URL is correctly saved to  the client and server configuration files, but the S3 uploader class computes an incorrect url, without the trailing "/cvmfs", as it only uses the S3 credential configuration file.

This should be fixed, allowing the use of familiar URLs for CVMFS repositories:

http://bucketname.s3.cern.ch/cvmfs/repo.cern.ch