make ssh port in alluxio.cli.validation.SshValidationTask configurable

When doing validateEnv command for a cluster, there will be a "Validating ssh.masters.reachable" and "Validating ssh.workers.reachable" stage which will check if the master can successfully ssh to the specify worker host

the method is "alluxio.cli.validation.SshValidationTask" which will hardcode the ssh port to 22

in typical Enterprise environment,for the sake of security, the default ssh port of 22 will usually change to a non-typical port

IMO, it is reasonable to have a configuration for the ssh port with a default value of 22