client.shared.utils_cgroup: Don't fail when cgclear fails

When system utils run under cgroups the cgclear command might fail.
Anyway the test is usually not affected by this so we can just notify
user via logging.WARN and proceed with testing.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
client.shared.utils_cgroup: Remove tailing '/' from cgroup root dir

Originally the root_dir was used directly, thus '/' was appended on init
time. The latest cgroup utils changed this behavior and append the
trailing '/' directly in the functions. Follow this behavior and don't
append this '/' in init time to synchronize those behaviors.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #733 from ldoktor/framework

client.shared.utils_cgoup: Two little modifications in utils_cgroup