Monitor: Add mode to generate confdb only
    
    With this mode we can add socket activated services and have systemd
    pre exec sssd to genrate the configuration file w/o starting the whole
    sssd if not necessary.
    
    https://fedorahosted.org/sssd/ticket/2243
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>