Refactor cgroups plugin

Move common files from DebianCgroups to Cgroups and drop separate
DebianPlugin class. Make add_copy_specs() invocations conform to
recommended indent style.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Update cgred configuration file path for RedHatCgroups

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>