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:  If these are going to be more-or-less static/hard-coded anyway, make them class-attributes and forget the generating methods.  Reason being, it's faster to read/understand the hard-coded definition than to "mentally parse" the method.