[OMPI devel] Change in hostfile behavior

[Ralph Castain]

Per an earlier telecon, I have modified the hostfile behavior slightly  
to allow hostfiles to subdivide allocations.

Briefly: given an allocation, we allow users to specify --hostfile on  
a per-app_context basis. In this mode, the hostfile info is used to  
filter the nodes that will be used for that app_context. However, the  
prior implementation only filtered the nodes themselves - i.e., it was  
a binary filter that allowed you to include or exclude an entire node.

The change now allows you to include a specified #slots for a given  
node as opposed to -all- slots from that node. You are limited to the  
#slots included in the original allocation. I just realized that I  
hadn't output a warning if you attempt to violate this condition -  
will do so shortly. Rather than just abort if this happens, I set the  
allocation to that of the original - please let me know if you would  
prefer it to abort.

If you have interest in this behavior, please check it out and let me  
know if this meets needs.

Ralph