release-ec2: add --image-size option

Add --image-size option its suggest fs_size_mb when build image

Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: KANATSU Minoru <icc.pot.tyew272@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>