roles/winbind: Use ad idmap backend

The `ad` identity mapper backend is apparently the only one that can
use shell, home directory, etc. attributes from the directory now (as of
Samba 4.6).

roles/winbind/defaults/main.yml

@@ -1,4 +1,4 @@
-winbind_idmap_range: 16777216-33554431
+winbind_idmap_range: 3000000-3009999
 winbind_nss_info: rfc2307
 winbind_use_default_domain: true
 winbind_offline_login: true