MesosZookeeperTest doesn't allow multiple masters

In order to test certain behavior of non-leading nodes - e.g. redirecting to the leading master when sending http api requests to a non-leading node - it would be helpful to be able to spin up multiple masters in the test. The ZooKeeperTest class should allow to do this, but fails when more than one master is being started. The test will run into a timeout when the second master is being started and exit with an error.