Fixed cgconfigparser to allow configs with no 'mount' section

Fixed cgconfigparser to allow configs with no 'mount' section

cgconfig service fails when something else mounts cgroup hierarchies during
boot (e.g. systemd). Therefore we should allow cgconfig.conf to have no
'mount' section -> it's up to admin to ensure that controllers are mounted as
needed.

Because 'group' section is already optional, with this patch cgconfigparser
will accept empty configuration file. This is probably the best default
config for distros with systemd.

Changelog:
  - fixed case with empty config file and no mounted controllers
  - reworked the if conditions to be more clear

Signed-off-by: Jan Safranek <jsafrane@redhat.com>