Inherit base image properties on instance creation

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

This is done by adding two utility functions that translate the image
metadata into system metadata and vice versa. These are  used from the
operations specified before to complement the image metadata and
preventing them to fail if the image doesn't exist.

The keys that the instance will inherit from the image are: 'min_ram',
'min_disk', 'disk_format' and 'container_format' together with the
inheritable image properties.

Fixes bug #1039662

Change-Id: If6a7b182a2fb46cdd05bd19760fcd246442b7f4b