SDAP: Fix handling of search bases
    
    We were rewriting the sdap_domain's search bases for only the first
    sdap_domain in the list, which does not work for subdomains.
    
    Also when search bases were already initialized in sdap_domain_subdom_add,
    we should only rewrite them when they were explicitly set in sssd.conf.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3351
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>