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.
Merge branch 'bugfix/verify-busy-dist-port-hang'