Inherit base image properties on instance creation

When starting an instance, a few properties from the image will be saved
to the image's instance_system_metadata with the prefix 'image_'.
This way, when creating an snapshot in the future we can preserve the
original properties even if the source image has been deleted or made
private.

The properties that the instance sytem_metadata will inherit are:

 * min_ram
 * min_disk
 * disk_format
 * container_format

Fixes bug #1039662

Change-Id: If6a7b182a2fb46cdd05bd19760fcd246442b7f4b