Fix check for existance of cgroups at creation

Fix check for existance of cgroups at creation

In the scenario where the cgroups were mounted but the
particular group did not exist, and the caller had not
requested auto-creation, the code would fail to return
an error condition. This caused the lxc_controller to
think the cgroup existed, and it then later failed when
attempting to use it

* src/util/cgroup.c: Raise an error if the cgroup path does not
  exist