Modify the osbuild configuration schema to accept only an array with a
single value as the `image_types`, in addition to a single string. The
single string was supported by the schema also before, but this fact was
not mentioned in the documentation, nor it was supported by the
`koji-osbuild` plugin of version lower than `9`.
Update the documentation accordingly.
Add unit test for invalid configuration containing more than one image
type.
Signed-off-by: Tomas Hozza <thozza@redhat.com>
805a1083a2