sdap: inherit SDAP_SASL_MECH if not set explicitly
    
    If ldap_sasl_mech is set for the configured domain in sssd.conf the
    value is inherited automatically to all sub-domains. The can be
    overwritten by setting ldap_sasl_mech for a given sub-domain explicitly
    in sssd.conf.
    
    Related to https://pagure.io/SSSD/sssd/issue/4006
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>