Ensure hierarchical roles work with old Mesos agents

If the Mesos master supports hierarchical roles but the agent does not, we need to ensure that we avoid putting the agent into a bad state, e.g., if the user creates a persistent volume.

One approach is to use an agent capability for hierarchical roles, and disallow creating persistent-volumes using a hierarchical role if the agent doesn't have the capability. We could also use an agent version check, although until MESOS-6975 is implemented, that will be a bit awkward.