Fix issue with mountCgroups not recognizing mounted cgroup

Changed mountCgroups to use the file /proc/mounts (instead of the
current case where it uses the less reliable
(http://www.mail-archive.com/lxc-devel@lists.sourceforge.net/msg00455.html)
/etc/mtab thorugh the mount program
Merge pull request #170 from piyushs/cgroup-fix

Fix issue with mountCgroups not recognizing mounted cgroup