utils_cgroup: fix the regexp in get_cgroup_mountpoint()

The last patch done by me has a bug in the regexp that it also takes
the " cgroup" or " type" from the cgroup mounts. This patch fix the
issue by updating the regexp.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
utils_cgroup: Fix get cgroup mount point

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>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Merge pull request #1253 from Antique/utils-cgroup

Utils cgroup