iotests: test external snapshot with bitmap copying

This test shows that external snapshots and incremental backups are
friends.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20190517152111.206494-3-vsementsov@virtuozzo.com
Signed-off-by: John Snow <jsnow@redhat.com>