Replaced fs.Stats() interface with fs.GetStats() which returns a cgroups.Stats strong type

Added a new method cgroups.GetStats() which will return a cgroups.Stats object which will contain all the available cgroup Stats.

Remove old Stats interface in libcontainers cgroups package.

Updated unit tests.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)