hub: mark baseurl as required in repo schema

Previously it the `repo` schema didn't mark the `baseurl` property as
required, although Cloud API "technically" requires it and the builder
code also made assumptions about it being always provided.

Note on Cloud API:
Cloud API requires that at least one of `baseurl`, `metalink` or
`mirrorlist` is specified for a repo. Since the plugin schema specifies
only `baseurl` as a property, it is as a result mandatory in order for
the Cloud API request to be valid and succeed.

Mark `baseurl` property as required in the `repo` schema and add unit
test for it.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

test/unit/test_hub.py

@@ -111,6 +111,24 @@ class TestHubPlugin(PluginTest):
                     ["arches"]
                 ],
                 "opts": {}
+            },
+            # repo without `baseurl` is not allowed
+            {
+                "args": [
+                    "name",
+                    "version",
+                    "distro",
+                    "image_type",
+                    "target",
+                    ["arches"]
+                ],
+                "opts": {
+                    "repo": [
+                        {
+                            "package_sets": ["set1", "set2"]
+                        }
+                    ]
+                }
             }
         ]