Add a way to specify a hostname or hdfs path in WeaveSpecification

I would like to be able to tell YARN through Weave API on what hosts to
allocate the containers or by specifying the hdfs path and Weave can deduce
the hosts via InputSplits.
Please let me know if there is already a way to do this. If there is not
I would like to add it myself. I could use some pointers just to get around quicker.