Aurora should allow users to create "privileged" containers

I'd like to allow users to set some configuration in their .aurora file which results in the "privileged" flag being set in ContainerInfo:
https://github.com/apache/mesos/blob/master/include/mesos/mesos.proto#L1017

If a user wants to, say, mount an NFS volume inside a container, "docker run" needs to be started in "priviledged" mode. See https://docs.docker.com/reference/run/#runtime-privilege-linux-capabilities-and-lxc-configuration

Like volume mount support there are security concerns here, so perhaps there needs to be a flag that allow administrators to disable this feature.

This requires mesos 0.21+