format: fix sapos node format at boot time

The bug was introduced in 57118615 and
was actually removing the main IPv6 of the machine because the
_addSystemAddress function is actually seeing the "tentative" attribute
for the IPv6 address added at boot time.

slapos/format.py

@@ -584,7 +584,11 @@ class Computer(object):
     ####################
     if alter_network:
       if self.address is not None:
-        self.interface._addSystemAddress(self.address, self.netmask)
+        self.interface.addIPv6Address(
+          partition_index=None,
+          addr=self.address,
+          netmask=self.netmask
+        )
 
       if create_tap and self.tap_gateway_interface:
         gateway_addr_dict = getIfaceAddressIPv4(self.tap_gateway_interface)