ldap: user get_ldap_conn_from_sdom_pvt() where possible
Currently when trying to prefer LDAP port lookups over Global Catalog
searches some AD specific structs are needed in the common LDAP
provider code. By using get_ldap_conn_from_sdom_pvt() this can be
avoided and all AD specific details are handled inside
sdap_async_initgroups_ad.c now.
Related to https://pagure.io/SSSD/sssd/issue/2474
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>