experiments: add instantiate method for NS3E2ENet

Subclasses can override to set up more state before the rest of the
initialization for running occurs.

experiments/simbricks/orchestration/simulators.py

@@ -954,8 +954,12 @@ class NS3E2ENet(NetSim):
                 self, e2e_sim.simbricks_component
             )
 
+    def instantiate(self):
+        pass
+
     def run_cmd(self, env):
         if self.first_run:
+            self.instantiate()
             for topo in self.e2e_topologies:
                 topo.add_to_network(self)