Forced mesos_pb2 to be generated in a flat fashion.

This fixes the regression caused by https://reviews.apache.org/r/23704/

Review: https://reviews.apache.org/r/24292
WIP: DockerInfo

Conflicts:
	src/docker/docker.cpp
	src/docker/docker.hpp
ContainerInfo

Conflicts:
	src/slave/containerizer/docker.cpp