Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/aws/exercises/network_load_balancer/exercise.md

@@ -0,0 +1,13 @@
+## AWS ELB - Network Load Balancer
+
+### Requirements                   
+
+Two running EC2 instances
+
+### Objectives
+
+1. Create a network load balancer 
+  1. healthy threshold: 3
+  2. unhealthy threshold: 3
+  3. interval: 10 seconds
+  4. Listener should be using TCP protocol on port 80