Resolve issue with verify_busy_dist_port test hanging indefinitely

The verify_busy_dist_port helper function cause_bdp:spam_nodes/1 recently
changed to be more aggressive in triggering busy_dist_port warnings. The
function changed to spawn 1 million processes to ensure the test generated
enough activity to trigger the warnings, but that number of processes exceeds
the 256 thousand process limit that is the Riak default. One consequence of
this can be that the rex server responsible for handling rpc calls can crash.
In some cases this leads to rpc calls by riak_test to shutdown the riak nodes
involved in the test to hang indefinitely. This change reduces the number of
processes spawned to 200 thousand. This should still be enough processes to 
trigger the busy_dist_port warnings, but without exceeding the beam process
limit.