Added support for CPU hard limits via CFS Bandwidth Control.

CFS is unique relative to existing Mesos cgroups support in that it
is a "subfeature" of an already supported cgroups subsystem, cpu.
Also, there are two "tunables" for configuring CFS bandwidth
limiting.

There are 4 approaches one could take:
1) Use the CFS bandwidth limiting if the feature is present.
2) Expose as separate flag, eg "cpu,cfs,memory,freezer".
3) Add feature flag support to subsystems via an additional delimiter,
 e.g., "cpu+cfs,memory,freezer".
4) Add an additional control flag via some other means.

Option 2's downside breaks the 1:1 mapping between cgroups subsystems
and a cgroups resource flag.

Option 3's downside is it greatly increases complexity of parsing
cgroups subsystem flags.

This diff takes option 1.

From: David Mackey <tdmackey@booleanhaiku.com>
Review: https://reviews.apache.org/r/9464

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1465819 13f79535-47bb-0310-9956-ffa450edef68