subtests.docker_cli.run_volumes_selinux: Remove libselinux dependency

Use `ls --scontext` instead of libselinux-python which might not be
present on all systems.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #391 from ldoktor/selinux_volumes

subtests.docker_cli.run_volumes_selinux: Remove libselinux dependency