Refactor EC2 image generation script. Now uses same procedure for both hvm and pv-grub

images. Root disks are now SSD backed and 20GB by default, both on hvm and pv-grub
(previously was 8GB for HVM). Added new eu-central region to the locations to copy images
to. Also the root disk for HVM instances was not deleted on termination with previous
images, this is fixed as well.