AD: Refresh subdomain data structures on startup
    
    Previously, if no changes were done to the list of subdomains, the SSSD
    didn't update its list of sdap_domain mappings for the new subdomain.
    This resulted in errors as no id_ctx was present for the subdomain
    during lookup.
    
    This patch moves the block of code performed during update to a function
    of its own and calls it during provider initialization as well.