Copy PYTHONPATH from installation env to hadoop-env.sh
Lower the acceptable count threshold in thread test to 5

The tests were sometimes failing on TravisCI (slower machines) with
counts below 100.  A test where we left the GIL locked showed that the
counter value doesn't change at all across calls, so in theory even an
increment of 1 would demonstrate that the mechanism is working.
Add PYTHONPATH to hadoop-env.sh during installation only if the variable is set