Support networks without gateway

Launching an instance when using network injection on a network
containing a subnet that has no gateway fails in scheduling with
a KeyError when using a legacy network_info provider and generates
an invalid network file for the others.

The following changes were done:

* The legacy transformer for network_info now always provides
  a gateway/gateway_v6 field, with a value of None if no gateway
  is available (this is similar to the other fields that are empty).
* The interfaces template was modified to include the gateway line
  only if a gateway is provided for the interface.

Fixes: bug #1207878
Change-Id: I887757a6bcba528059ace8831a7d23a31b08c630

nova/virt/interfaces.template

@@ -13,7 +13,9 @@ iface ${ifc.name} inet static
         address ${ifc.address}
         netmask ${ifc.netmask}
         broadcast ${ifc.broadcast}
+#if $ifc.gateway
         gateway ${ifc.gateway}
+#end if
 #if $ifc.dns
         dns-nameservers ${ifc.dns}
 #end if
@@ -22,7 +24,9 @@ iface ${ifc.name} inet static
 iface ${ifc.name} inet6 static
     address ${ifc.address_v6}
     netmask ${ifc.netmask_v6}
+#if $ifc.gateway_v6
     gateway ${ifc.gateway_v6}
 #end if
+#end if
 
 #end for