Main addition is the "subcontainers" resource which provides
ContainerInfo for all subcontainers (recursively and including
self) of the specified container.
This allows a ContainerHandlerFactory to register a CanHandle() function
which is called to determine whether the factory can handle a particular
container.
This commit disables being able to run cAdvisor without lmctfy. This
should be enabled again with a "no-op" global factory which I would like
to do in a separate PR.
