MesosContainerizer launch helper should take ContainerLaunchInfo.

Currently, the launch helper takes various flags from MesosContainerizer to launch the container. This makes it very hard to add more parameters to the launch helper. To simplify that, MesosContainerizer can pass 'ContainerLaunchInfo' to the launch helper instead. 'ContainerLaunchInfo' is also the protobuf message returned by isolators during 'prepare()'. This makes it very easy to merge them and send it to the launch helper. More importantly, this makes it very easy to add more parameters to the launch helper in the future.