Use monitor flag by default in cluster start scripts

The -m flag makes the start command synchronous, waiting until everything it starts is up. This is a good default behavior since new users will often assume that when start returns, Alluxio will successfully start. We can use a different flag to disable startup monitoring, in case users prefer the original behavior of fire and forget.

This is a backwards compatibility breaking change, so we should wait until Alluxio 2.0 and call out the change.