utils_cgroup: Strip the slash from cgroup path

We have to strip the slash from cgroup path because the "join" function
takes the last element starting with slash as the root path.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
utils_cgroup: Fix regexp for cgroup controllers

On some systems the cpu and cpuacct controllers are mounted together
like cpu,cpuacct and we should count with it.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>