ftests/cgroup.py: add support to list shared mount point

ftests/cgroup.py: add support to list shared mount point

Cgroup::get_cgroup_mounts(), could create a list of mount points and in
the case of shared mount points such as cpu,cpuacct in the cgroup v1. It
would only add a mount point for the last of the two controllers listed
as the mount point, for example:
cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpu,cpuacct 0 0

mount list will look over cpu controller.  Add support to recognize and
add both controllers to the list of mount points.

Reported-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>