subtests.docker_cli: Add run_env test

This test contains workaround using InteractiveAsyncDockerCmd due of https://bugzilla.redhat.com/show_bug.cgi?id=1131592 and it fixes the issue https://github.com/autotest/autotest-docker/issues/252

this tests the ENV related features. Currently it only contains one
subsubtest, which is focused on linked containers. It sends data
using the addr from ENV to the server, which resends them back.

In the end it checks the valid data + ENV variables added via `-e x=y`
including the linked values ($linked_ENV_...)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
I don't think you need to have `_stdin` public. It's related to the command itself, so calling `dkrcmd.close()` makes sense to me. Anyway you told us to not update API so that's why everything goes into tests themselves. If you want, we can combine all existing versions and create interactive container class (I guess @jzupka will have his requirements too)