select/cray: do not override the node reason field

With the current configuration, setting DownNodes in slurm.conf was not possible,
since node_ptr->reason gets overwritten by basil_get_initial_state().

The patch updates setting the initial state so that
 * initial 'reason' fields remain untouched;
 * a new 'reason' is set only if 
   - the node is not already recognized as down or
   - no reason has been set so far;
 * it frees any previously set 'reason' if the node is allocated or idle.

This code has been tested to work while we were waiting for a missing replacement
blade (marked as 'DownNodes' in slurm.conf).