routing_table: Deduplicate rebuilds

We don't need to rebuild the routing table for every single interface
and system configuration object. It now only rebuilds the table once
every refresh of the object tree.

Change-Id: I6cc1cfa1ee6ad17307e3e9c7a2eaa352675ba7e8
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/network_manager.cpp b/src/network_manager.cpp
index e2d48f1..48e3848 100644
--- a/src/network_manager.cpp
+++ b/src/network_manager.cpp
@@ -167,6 +167,8 @@
 
 void Manager::createChildObjects()
 {
+    routeTable.refresh();
+
     // creates the ethernet interface dbus object.
     createInterfaces();