Mesos containerizer doesn't reject tasks with container info set

DockerContainerizer doesn't accept tasks that do not have TaskInfo.ContainerInfo set, but MesosContainerizer accepts tasks even if TaskInfo.ContainerInfo is set. It should not.

This means, currently if a slave has to support both docker and non-docker tasks, the order of containerizers in --containerizers is important, viz., "docker,mesos" works but "mesos,docker" doesn't work when running a non-docker task.