subtests.docker_cli.import_export: Add with_env variant

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>
Merge pull request #390 from ldoktor/import_export

subtests.docker_cli.import_export: Add with_env variant