node_features/knl_cray use cnselect rather than capmc

node_features/knl_cray plugin modified to get current node MCDRAM
   mode using cnselect command rather than capmc command. This gets the
   current BIOS setting, which could differ from what is configured to be
   set on boot, at least if something other than Slurm is modifying
   KNL modes.
bug 2850