Ties quantum, melange, and nova network model

get_instance_nw_info() now returns network model, and keeps the network
info cache up to date.
virt shim and translation in place for virts to get at the old stuff

Change-Id: I070ea7d8564af6c644059d1c209542d250d19ddb

nova/virt/driver.py

@@ -605,3 +605,10 @@ class ComputeDriver(object):
         Note that this function takes an instance ID.
         """
         raise NotImplementedError()
+
+    def legacy_nwinfo(self):
+        """
+        Indicate if the driver requires the legacy network_info format.
+        """
+        # TODO(tr3buchet): update all subclasses and remove this
+        return True