reproduction test for basho/riak_kv#475 (MR reduce with node down)

This test reproduces the case described in basho/riak_kv#475 by starting four nodes, killing one, loading some data, then MapReducing that data. The query expects to produce a count of the objects in the bucket, but occasionally produces `0` because of the lack of node liveness consideration while planning where to process the reduce phase.

This test is based off one submitted by Alexander Gunin to the
riak-users mailing list as an issue reproducer.
[http://lists.basho.com/pipermail/riak-users_lists.basho.com/2013-January/010896.html]