ec2-upload: Fix batch upload with small instances

Due to a global variable in image type definition code
small instance directive becomes sticky and applies automatically
to all further images of the batch.

Fixed by making the variable local.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>