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>
Yeah, after thinking more, close() is probably fine, though direct access could be handy.  Maybe add a ``stdin`` property that returns a python file object or something?

Not entirely true, **changes** to the API are not allowed for revision releases, however small additions/fixes are always allowed as long as they don't break any already merged tests.  However,  looking at my current priorities, yes you're right, we probably should just put this in as-is, even if duplicated, so be it.  We need these tests merged and passing more than we need perfect API's, we can work on tidying up these things in the next version.