osbuild: accept only a single image type in the configuration
    
    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>