Since is possible to create images with no
ramdisk_id, kernel_id parameters on glance v1,
then the schema-image.json needs to be modified to
allow null, string values for both attributes
and prevent LP #1447193 to happen with a glance v2
client.
Change-Id: I1319a1f6bdb69db90f9be7cf34df62cbd8538969
Closes-Bug: #1516330
Related-Bug: #1447193