SSSD: Allow refreshing only certain section with --genconf
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3862
    
    Adds a new option --genconf-section for the sssd binary.  If this new
    option --genconf-section is used, then only the section passed as this
    option's value is refreshed.
    
    Conversely, if this section no longer exists in the config file, then it
    is removed from the confdb
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>