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>
(Haven't looked at rest of code below yet) Are you absolutely certain we need this import?

Reason is:  Adding any external dependencies implies changes to test-grids, which can sometimes take a long time or be denied.  That could hold the test "hostage" for grid-updates - meaning it simply doesn't get tested.

Some suggestions:

*  Bundle the dependent code into dockertest.
*  Instead of raising ``TestNAError``  Define some work-alike classes/functions that allow the test to perform.
*  Get the dependencies secured in test-grids, then write/merge the test.
*  Work out an alternate way to test w/o the dependency.

Otherwise, if this module is required for general SELinux-targeted support on our target platform, drop a comment here to that effect, so then there's no question :smile: