Hub: support image_type being an array for backwards compatibility

The support for specifying multiple `image_types` for a single compose
has been removed by [1]. This turned out to be problematic, because e.g.
Pungi uses the array type when triggering image builds via osbuild.

Bring back the support for specifying the `image_type` as an array, but
restrict it to a single item. This will cover the Pungi use-case, since
it is always passing a single `image_type` in the array. The array is
then converted to a string in the Hub plugin and passed as such to the
Builder plugin.

Extend unit tests covering the introduced compatibility layer.

[1] c725265081

plugins/hub/osbuild.py

@@ -28,8 +28,21 @@ OSBUILD_IMAGE_SCHEMA = {
             "description": "Distribution"
         },
         {
-            "type": "string",
-            "description": "Image Type",
+            "oneOf": [
+                {
+                    "type": "string",
+                    "description": "Image Type"
+                },
+                {
+                    "type": "array",
+                    "description": "Image Types (this option is deprecated)",
+                    "minItems": 1,
+                    "maxItems": 1,
+                    "items": {
+                        "type": "string"
+                    }
+                }
+            ]
         },
         {
             "type": "string",
@@ -224,6 +237,12 @@ def osbuildImage(name, version, distro, image_type, target, arches, opts=None, p
     except jsonschema.exceptions.ValidationError as err:
         raise koji.ParameterError(str(err)) from None
 
+    # Support array for backwards compatibility
+    # This check must be done after the schema validation
+    if isinstance(image_type, list):
+        image_type = image_type[0]
+        args = [name, version, distro, image_type, target, arches, opts]
+
     if priority and priority < 0 and not context.session.hasPerm('admin'):
         raise koji.ActionNotAllowed('only admins may create high-priority tasks')