Refactored cgroups tests and made the root cgroup
used by cgroups isolation module configurable.

From: Vinod Kone <vinodkone@gmail.com>
Review: https://reviews.apache.org/r/9814

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1454620 13f79535-47bb-0310-9956-ffa450edef68