Automatic merge from submit-queue

Fix detection of docker cgroup on RHEL

Check docker's pid file, then fallback to pidof when trying to determine the pid for docker. The
latest docker RPM for RHEL changes /usr/bin/docker from an executable to a shell script (to support
/usr/bin/docker-current and /usr/bin/docker-latest). The pidof check for docker fails in this case,
so we check /var/run/docker.pid first (the default location), and fallback to pidof if that fails.

@kubernetes/sig-node @kubernetes/rh-cluster-infra