[SPARK-13414][MESOS] Allow multiple dispatchers to be launched.

## What changes were proposed in this pull request?

Users might want to start multiple mesos dispatchers, as each dispatcher can potentially be part of different roles and used for multi-tenancy.

To allow multiple Mesos dispatchers to be launched, we need to be able to specify a instance number when starting the dispatcher daemon.

## How was the this patch tested?

Manual testing

Author: Timothy Chen <tnachen@gmail.com>

Closes #11281 from tnachen/multiple_cluster_dispatchers.