contrib: XtreemFSHadoopClient: Add support for multiple volumes.
contrib: XtreemFSHadoopClient: extend support for mutliple volumes.
Now all available volumes can be used within hadoop, the default volume is specificed by xtreemfs.defaultVolumeName in core-site.xml.
tests: Enable Hadoop test
tests: Update hadoop test to new XtreemFSHadoopClient config parameter
contrib: XtreemfsHadoopClient: Update Exception to new XtreemFSHadoopClient config parameter