subtests.docker_cli.import_export: Add with_env variant

Fixes the https://github.com/autotest/autotest-docker/issues/373

Tests the ``docker import --env`` feature by setting multiple variables,
exporting container, importing it and running it while overriding some
of the variables using various ways to overwrite them. (import cmdline,
import file, run cmdline, ...)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Suggestion: Make this a ``@staticmethod`` in the ``subsubtest`` class in case there's a derived class that wants to override it or use it in another way.