CP-311169: samba: include /etc/samba/smb.extra.conf (#6871)

Samba has lots of feature and configurations,
https://www.samba.org/samba/docs/current/man-html/smb.conf.5.html

Some of them are passthrough from xapi configuration to samba However,
this requires xapi rebuild and make xapi uncessary complicated.

On the other side, sometimes we do need customer to tune some args,
especially for debug purpose, and we do want such configuration can
persist cross xapi restart.

A new configuration /etc/samba/cust.conf is introduced. The xapi
generated/overrided /etc/samba/smb.conf include it. Customer can update
this file with guide of support team for debug purpose

Author: liulinC

ocaml/xapi/extauth_plugin_ADwinbind.ml

@@ -780,6 +780,7 @@ let query_domain_workgroup ~domain =
 
 let config_winbind_daemon ~workgroup ~netbios_name ~domain =
   let smb_config = "/etc/samba/smb.conf" in
+  let extra_conf = "/etc/samba/smb.extra.conf" in
   let string_of_bool = function true -> "yes" | false -> "no" in
 
   let scan_trusted_domains =
@@ -788,7 +789,8 @@ let config_winbind_daemon ~workgroup ~netbios_name ~domain =
   ( match (workgroup, netbios_name, domain) with
     | Some wkgroup, Some netbios, Some dom ->
         [
-          "# autogenerated by xapi"
+          Printf.sprintf "# This file is managed by xapi, update %s instead"
+            extra_conf
         ; "[global]"
         ; "client use kerberos = required"
         ; "sync machine password to keytab = \
@@ -815,6 +817,7 @@ let config_winbind_daemon ~workgroup ~netbios_name ~domain =
         ; "idmap config * : backend = autorid"
         ; Printf.sprintf "idmap config * : range = %d-%d" 2_000_000 99_999_999
         ; Printf.sprintf "log level = %s" (debug_level ())
+        ; Printf.sprintf "include = %s" extra_conf
         ; "" (* Empty line at the end *)
         ]
     | _ ->