nova docker driver cannot find cgroup in /proc/mounts on RHEL

In order to support the docker hypervisor on rhel/centos or fedora
we had to adapt the function which resolves the cgroup path.

Change-Id: Ie24028f822d91d65d4f90a7941e7b11874a3b425
Closes-Bug: #1243101
Merge "nova docker driver cannot find cgroup in /proc/mounts on RHEL"