Don't assume that an enabled login plugin is available
    
    For upgrades or other database corruption use a safer
    method of getting a value from the list of available
    login plugins.
    
    This function can safely return None upon which the user
    will be presented a login page with a list of mechanisms
    to choose from.
    
    https://fedorahosted.org/ipsilon/ticket/131
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>