utils_cgroup: Fix get cgroup mount point

The new get_cgroup_mountpoint function resolve ordinary cgroup mount
points such as 'memory' to '/sys/fs/cgroup/memory cgroup'.

To fix this, I use explicit code instead of regex to match cgroup
mountpoint.

Checking get_all_controller is no longer needed since the controller
must be invalid if it's not in mount table.

get_all_controller in another function is also not needed because it is
followed by a get_cgroup_mountpoint which will raise exception if
controller is invalid.

Unit test is also added for get_cgroup_mountpoint.

Signed-off-by: Hao Liu <hliu@redhat.com>