Updated 'containers' implementation to forward to the ECP.

Updates 'containers' implementation within the ExternalContainerizer
to forward the call to the ECP.  Adds new 'containers' proto message
to containers.proto.

Some minor cleanups like removing of needless namespace mentions that
slipped through.

NOTE: The test_containerizer.py implementation of 'containers' is
incomplete, it merely poses as a mock-up.  NOTE: This does *not* fix
MESOS-1257. A complete test_containerizer C++ implementation is
upcoming to fix these notes.

Review: https://reviews.apache.org/r/21040