Don't pass the MBS API URL around as a parameter
    
    Rather than making it a parameter, make it a property of the Commands
    object that the caller needs to set first. This gives more flexibility
    when using the API URL - in particular it can be use when loading a
    property value. We still set it only for MBS commands to avoid validating
    the MBS API configuration and checking the API version when MBS is not
    being used.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>