Allow ramdisk_id, kernel_id to be null on schema

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.

APIImpact

Closes-Bug: #1447215
Related-Bug: #1447193

Change-Id: I645c797553ccaa84062f9ad28f2349ae738a0c96
Signed-off-by: Jorge Niedbalski <jorge.niedbalski@canonical.com>

etc/schema-image.json

@@ -1,11 +1,11 @@
 {
     "kernel_id": {
-        "type": "string",
+        "type": ["null", "string"],
         "pattern": "^([0-9a-fA-F]){8}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){12}$",
         "description": "ID of image stored in Glance that should be used as the kernel when booting an AMI-style image."
     },
     "ramdisk_id": {
-        "type": "string",
+        "type": ["null", "string"],
         "pattern": "^([0-9a-fA-F]){8}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){12}$",
         "description": "ID of image stored in Glance that should be used as the ramdisk when booting an AMI-style image."
     },