Merge pull request #1059 from datawolf/use-WriteCgrougProc

cgroup: using WriteCgroupProc to write the specified pid into the cgroup's cgroup.procs file

Author: Mrunal Patel

libcontainer/cgroups/fs/cpu.go

@@ -46,7 +46,7 @@ func (s *CpuGroup) ApplyDir(path string, cgroup *configs.Cgroup, pid int) error
 	}
 	// because we are not using d.join we need to place the pid into the procs file
 	// unlike the other subsystems
-	if err := writeFile(path, "cgroup.procs", strconv.Itoa(pid)); err != nil {
+	if err := cgroups.WriteCgroupProc(path, pid); err != nil {
 		return err
 	}