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>
Interesting, so ``BgJob`` kind of hides this problem, great! :cry: Thanks for the explanation though, it is a tricky situation.  

One simple/nieve option could be to make ``_stdin`` public attribute, and get rid of the ``close()`` method all together.  The calling code is in a much better position to judge how to finalize anyway.  Then in the future, we can always add in the method if it makes sense and not break any existing interfaces.  It's not a perfect general solution, just a small seed to grow on later and save putting the same code into 2+ tests in 0.7.7 (my main motivation).  Could that be made to work?