Don't use dhcp4 for none mode (xapi-project#6001)

For the context, we had an strange behavior where an host installed with
IPv6 stack only got an Ipv4 address only during the first boot due to
the start of a systemd service called chrony-wait. This is because
during the first boot the **xcp-networkd** looks for a configuration
from `/etc/firstboot.d/data/management.conf` and if the `MODE=none` it
uses DHCP4. And as chrony-wait is started it got a source in IPv4 before
XAPI had time to reconfigure it using IPv6 stack. So we think that if
the MODE is none we shouldn't use DHCP4.

Regards,
Guillaume

ocaml/networkd/lib/network_config.ml

@@ -135,9 +135,12 @@ let read_management_conf () =
           in
           let dns = (nameservers, domains) in
           (Static4 [(ip, prefixlen)], gateway, dns)
-      | "dhcp" | _ ->
+      | "dhcp" ->
           (DHCP4, None, ([], []))
+      | _ ->
+          (None4, None, ([], []))
     in
+
     let phy_interface = {default_interface with persistent_i= true} in
     let bridge_interface =
       {default_interface with ipv4_conf; ipv4_gateway; persistent_i= true; dns}