describe-images: add fields 'platform' and 'root_device_type' even if empty
describe-images: add fields 'platform' and 'root_device_type' even if empty

This prints out 'platform' and 'root_device_type' always, rather than only
if they are non-empty.

It also prints empty string rather than " " for each of the following:
   image.architecture
   image.type
   image.kernel_id
   image.ramdisk_id
   image.platform
   image.root_device_type

vasiliykochergin/euca2ools,gholms/euca2ools,nagyistoce/euca2ools,vasiliykochergin/euca2ools,gholms/euca2ools,nagyistoce/euca2ools,jhajek/euca2ools,jhajek/euca2ools