dockertest: Moves SubBase to separate module to avoid circular dependencies.

It allows subtest module use more free without circular dependencies
with other dockertest core modules.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
dockertest: Adds checking of SubBase in contructors for images and container

Keep it consist with dockercmd.

Signed-off-by: Jiří Župka <jzupka@redhat.com>