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>
There are 2 main problems.
1) When you execute container and close the STDIN pipe, the process dies (while container is still running in the background)
2) When the container finishes, the process stays alive returning None until you write into stdin (not necessarily '\n').

These issues are connected but different. When you use default BgJob stdin, it sends '\n' and closes the STDIN. If the container manages to finish prior to this close, you won't see any failures and you'll get the whole output. Anyway if you execute longer-term program, the process is interrupted by STDIN close and you'll get only partial output.

Alternatively if you use `InteractiveAsyncDockerCmd`, you need to decide when the process is finished. Because as you still have the STDIN opened, the process never finishes until you send data to STDIN of the finished container. So I can use this manually in some tests when I know that the command finished and I'll manually send the data afterwards or I just close the STDIN when I believe it finished.

The only way to create generic workaround is to periodically send some data to STDIN, but this affects the output (yes, you can send some \0x0* which might be invisible, but still it might affect the program underneath).

These are the reasons why generic workaround is kind-of complicated and it'd be better to push developers to fix the Go issue (because this was traced down to be Go-lang issue, which is rotting in tracer more than half of year without any change).