OMPI: btl openib: fix detection of max registarable memory
    
Deal with the case when mlx4 module is loaded but device
is not present

cmr=v1.8.3:reviewer=miked
OMPI/COLL/Tuned: add command line params for thresholds to decide if small/intermediate MSGs alltoall algorithm will be used.

cmr=v1.8.3:reviewer=miked