If the default hostfile is empty, we need to pickup all the known nodes, not just the head node.

cmr:v1.7.3:reviewer=jsquyres
cmr:v1.6.6:reviewer=jsquyres
Even if we don't find the default hostfile, and nothing else was provided, then use all the known nodes.

cmr:v1.7.3:#3653:reviewer=jsquyres
cmr:v1.6.6:#3654:reviewer=jsquyres