add a new option 'env.cgrouppath' to lxc_proc

 * Set the path where 'tasks' sysfs files are stored, default: empty
 * In some situations, LXC creates tasks files at multiple locations, but one of them has correct process numbers
Merge pull request #4 from mittyorz/lxc_proc

add a new option 'env.cgrouppath' to lxc_proc