HPCC-10920 Return ClusterName in WsDFU.DFUInfo response

If a logical file is associated with only one cluster, the cluster
name is returned in WsDFU.DFUInfo response. If a logical file is
associated with multiple clusters, the cluster names are returned
inside a comma separated string in the response.

Signed-off-by: Kevin Wang kevin.wang@lexisnexis.com