Enforce that DiskInfo principal is equal to framework/operator principal

Currently, we require that ReservationInfo.principal be equal to the principal provided for authentication, which means that when HTTP authentication is disabled this field cannot be set. Based on comments in 'mesos.proto', the original intention was to enforce this same constraint for Persistence.principal, but it seems that we don't enforce it. This should be changed to make the two fields equivalent, with one exception: when the framework/operator principal is None, we should allow the principal in DiskInfo to take any value, along the same lines as MESOS-5212.