Add RootFs field to configuration options in libcontainer's Config

Since currently the container.json file does not include the pathname
to a container's root filesystem, we need to parse /proc/mounts which
is slow and error-prone.  This patch addresses this issue by adding a
new RootFs field.

Signed-off-by: Saied Kazemi <saied@google.com>
Sure, I reworked the code and changed the calls.