add support for Mesos masters running with --no-root_submissions, see AURORA-311

Review board entry for https://issues.apache.org/jira/browse/AURORA-311

Testing Done:
Tested successfully on internal cluster with version 0.5.1. Aurora was able
to connect to a mesos master that was running with --no-root_submissions and
was able to schedule tasks with a role of EXECUTOR_USER that then were
running as the specified executor user.

Bugs closed: AURORA-311

Reviewed at https://reviews.apache.org/r/23471/