Add a command to check Alluxio environment
It would be helpful to have an Alluxio shell command that do a bunch of environment checks to ensure Alluxio can start correctly. It might include the following checks:
Check Alluxio master port is open and available
Check Alluxio worker port is open and available
Check nodes specified in conf/worker can be reached through ssh.
The check should run without requiring Alluxio being started.