tests: Add functional test cgroup class

tests: Add functional test cgroup class

This commit adds a Cgroup() class for managing cgroups.  This
class provides static methods for interacting with libcgroup's
interfaces including cgset, cgget, cgcreate, etc.

Example usages:
  # create a cgroup in the cpuset controller named foo
  Cgroup.create(config, 'cpuset', 'foo')

  # set cpu.shares for foobar to 500
  Cgroup.set(config, 'foobar', 'cpu.shares', '500')

  # get the limit_in_bytes for AnotherCgroup.  Have libcgroup
  # strip off all of the decorations so that only the value is
  # returned
  limit_in_bytes = Cgroup.get(config, controller=None,
      cgname='AnotherCgroup', setting='memory.limit_in_bytes',
      print_headers=False, values_only=True)

Providing invalid parameters to a Cgroup method will result in
a ValueError while a failure to execute a command will result
in a RunError.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>