subtests.docker_cli: Add new test run_volumes_selinux

This test uses `docker run --volume ::Z/z` and checks it sets the
selinux context properly.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
My RHEL7 VM minimal-install did **not** include ``python-selinux``, and there's a chance the Atomic host both doesn't include it, and it can't be installed (it's read-only).  So having this as a dependency means this test may not work under Atomic.  Since you're only using it for a single call (below) which just retrieves an extended-attribute value, it's probably of greater overall benefit to just duplicate that functionality minus the dependency.  The ``ls`` thing I showed was just a cheap example, since it's built-in and doesn't rely on any extra packages.  I'm sure there are other ways to retrieve the value as well.