subtests.docker_cli.import_export: Add with_env variant

Tests the ``docker import --env`` feature by setting multiple variables,
exporting container, importing it and running it while overriding some
of the variables using various ways to overwrite them. (import cmdline,
import file, run cmdline, ...)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #390 from ldoktor/import_export

subtests.docker_cli.import_export: Add with_env variant
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>
Merge pull request #388 from ldoktor/selinux_volumes

subtests.docker_cli: Add new test run_volumes_selinux
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