Add ComposeInfo fields to models.Compose.
    
    - The `Compose.create()` method now accepts ComposeInfo instance and stores
      its metadata into `Compose` model.
    - The `Compose.create()` also bumps the `respin` number in case the same
      Compose already exists in the database.
    - The `Compose.json()` returns the ComposeInfo metadata.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>