[nantes] add the warranty extension for service nodes of nantes

input/grid5000/sites/nantes/servers/servers.yaml

 srv:
   kind: physical
   alias: [adminfront, af]
-  warranty: 11.2017
+  warranty: 11.2019
   serial: HP2YG5J
   network_adapters:
     default:
@@ -14,7 +14,7 @@ srv:
 srv-data:
   kind: physical
   alias: [nfs, home-g5k, public]
-  warranty: 11.2017
+  warranty: 11.2019
   serial: 346ZG5J
   network_adapters:
     default:
@@ -28,7 +28,7 @@ srv-2:
   kind: physical
   alias:
     - ntp
-  warranty: 11.2017
+  warranty: 11.2019
   serial: JP2YG5J
   network_adapters:
     default: