Use default topology when no topology file is provided

Change-Id: Icbc434a82cc147c84680c77e2f089f20d50ad803
refs: #2915
diff --git a/ndn_utils/topologies/default-topology.conf b/ndn_utils/topologies/default-topology.conf
new file mode 100644
index 0000000..9452122
--- /dev/null
+++ b/ndn_utils/topologies/default-topology.conf
@@ -0,0 +1,9 @@
+[nodes]
+a: _
+b: _
+c: _
+d: _
+[links]
+a:b delay=10ms
+a:c delay=10ms
+b:d delay=10ms