Merge autotest and virttest cgroup_utils library

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
client.shared.utils_cgroup: Apply modifications from virttest version

Apply modifications which were accepted in virttest version of this
library.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #683 from ldoktor/framework

Merge autotest and virttest cgroup_utils library