Nested container's launch command is not set correctly in docker/runtime isolator.

We in-correctly append executor's command to the default entrypoint of the image:
https://github.com/apache/mesos/blob/1.1.0/src/slave/containerizer/mesos/isolators/docker/runtime.cpp#L323-L327
https://github.com/apache/mesos/blob/1.1.0/src/slave/containerizer/mesos/isolators/docker/runtime.cpp#L350-L354

This is probably due to the negligence while adding nested container support.