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>