Resolve an unexpected behavior in hostfile allocations. Now that we filter allocations to determine what will be used for mapping, let the initial global pool be the union of nodes from all sources (default hostfile, hostfiles, and dash-hosts). Each app will filter down to only those specified for it using its own hostfile and dash-host options.

cmr=v1.7.4:reviewer=jsquyres:subject=Resolve an unexpected behavior in hostfile allocations
Remove an annoying warning. If the user excludes a non-existent interface, there is no reason to warn - the interface may simply not exist on that node.

cmr=v1.7.4:reviewer=jsquyres:subject=Remove an annoying warning