cleanup cvmfs_server changes
FIX: create backend storage only if needed
add is_s3_upstream() to cvmfs_server
`cvmfs_server check` is S3 aware
show storage type in `cvmfs_server list`
add `cvmfs_server mkfs -s` to create S3 repos
`cvmfs_server add-replica -s` for S3 based Stratum1
remove S3 stratum1 support for now
Merge pull request #471 from reneme/feature-s3_server_integration

Feature: S3 Server Integration