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>
It's not that simple to fix it for all cases. You could use `InteractiveAsyncDockerCmd` for all `DockerCmd` commands executed with string, but then there is no way to discover, when the command finishes (due of another bug when the container finishes you need to send another character to STDIN to make it finish, otherwise the process still lives). So there is no general solution and this works only because I know that it's going to end and ~ when. (and it doesn't work all the time... I mean you can send '\n' every period of time, but it would spam the output...