format: Try hard to keep the same computer address

self.interface.getGlobalScopeAddressList which uses netifaces can have
different sorting of the addresses depending of the version, so prefer the
already configured IPv6 address.

slapos/format.py

@@ -301,7 +301,16 @@ class Computer(object):
         if netaddr.valid_ipv6(address['addr']):
           computer_partition_address_list.append(address['addr'])
     # Going through addresses of the computer's interface
-    for address_dict in self.interface.getGlobalScopeAddressList():
+    available_address_list = self.interface.getGlobalScopeAddressList()
+    # First scan: prefer the existing address, as the available_address_list
+    #             might be sorted differently
+    for address_dict in available_address_list:
+      if self.address == address_dict['addr']:
+        if self.address not in computer_partition_address_list:
+          return address_dict
+
+    # Second scan: take first not occupied address
+    for address_dict in available_address_list:
       # Comparing with computer's partition addresses
       if address_dict['addr'] not in computer_partition_address_list:
         return address_dict