cloudapi/v2: ensure only one image per a compose in the API spec

We've never had the ability to build multiple images per a compose, this
commit thus rips out support for this on the API level:

image_requests is now image_request and it accepts only one ImageRequest
object instead of an array of them.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/cloudapi/v2/openapi.v2.yml

@@ -474,15 +474,13 @@ components:
       - type: object
         required:
           - distribution
-          - image_requests
+          - image_request
         properties:
           distribution:
             type: string
             example: 'rhel-8'
-          image_requests:
-            type: array
-            items:
-              $ref: '#/components/schemas/ImageRequest'
+          image_request:
+            $ref: '#/components/schemas/ImageRequest'
           customizations:
             $ref: '#/components/schemas/Customizations'
     ImageRequest: