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>
Hmm, actually, yeah, thinking more, this little thing is clean, simple, and a tried-and-true pattern for us.  How about  just toss it into ``dockertest.dockercmd``?  Then we don't need to call it a "workaround" and fix it inside every test.  Sound good?