Restore data locality preference for hadoop and spark

[jimczi]

The data locality preference has been dropped in #812. We should restore the functionality but instead of setting one node per partition we should let hadoop/spark choose among the nodes that host the index/shard targeted by the partition. Since it's only a preference, having multiple hosts that could serve the query is beneficial in terms of execution. The list of hosts should be shuffled in order to spread the execution of multiple partitions that target the same index/shard.