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>

plugins/hub/osbuild.py

@@ -69,6 +69,7 @@ OSBUILD_IMAGE_SCHEMA = {
             "title": "Repository options",
             "type": "object",
             "additionalProperties": False,
+            "required": ["baseurl"],
             "properties": {
                 "baseurl": {
                     "type": "string"