restapi: Don't validate missing cloud-init enums

Currently we blindly try to validate enums included in cloud-init
specifications, but they may be missing. As they aren't declared as
mandatory this patch changes the validator so it won't try to validate
them when they are missing.

Change-Id: If7c29a92909fb9f436b06d2ed534fe2cbc34a293
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>