ipatests: replace utility for editing sssd.conf
    
    There are three patterns for editing sssd.conf in tests now:
    1. using modify_sssd_conf() which allows to modify only domain sections
    2. using remote_ini_file
    3. direct file editing using `sed`
    
    This patch introduces new utility function which combines advantages of
    first two approaches:
    * changes are verified against schema, so that mistakes can be spotted
      early
    * has convenient interface for simple options modification,
      both in domain and service sections
    * allows sophisticated modifications through SSSDConfig object
    
    Fixes: https://pagure.io/freeipa/issue/8219
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>