CLUSTER-5. bento-cluster should not start if any ports are taken.

This commit modifies bento-cluster so that it does not start if any of the
configured ports are taken. To facilitate this, a new tool was added which
checks that all the ports are open. If the check fails, the bento script refuses
to start the clusters.

Signed-off-by: Aaron Kimball <aaron@wibidata.com>