* Closes issue #261 on node_exporter.
Delegated mdstat parsing to procfs project. mdadm_linux.go now only exports the metrics.
-> Added disk labels: "fail", "spare", "active" to indicate disk status
-> hanged metric node_md_disks_total ==> node_md_disks_required
-> Removed test cases for mdadm_linux.go, as the functionality they tested for has been moved to procfs project.
Signed-off-by: Advait Bhatwadekar <advait123@ymail.com>