Per the OMPI devel conference that changed our default behaviors:

* default to bind-to core 
* map-by slot if np=2
* map-by socket (balance across sockets on each node) if np > 2
* map-by <obj> will imply rank-by <obj> by default (leave default binding as above) 

Fix a bug in the map-by <obj> mapper where we incorrectly compute the #procs to assign if the #slots > #procs

cmr=v1.7.4:reviewer=jsquyres:subject=Update default binding and mapping values