Add a finished method to spokes (#1300499)
    
    This method can be used to clean up a spoke before starting the
    installation. The method is called when continue is clicked on the
    summary hub. It is completely optional and if it doesn't exist on the
    spoke it will be skipped.
    
    Add finished method to the Network spoke to handle killing nmce and
    disconnecting callbacks so that they don't cause problems if the
    installation triggers network changes.