Rename exercises dir

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

topics/aws/exercises/elastic_network_interfaces/exercise.md

@@ -0,0 +1,10 @@
+## AWS EC2 - Elastic Network Interfaces
+
+### Requirements
+
+* An EC2 instance with network interface
+
+### Objectives
+
+A. Create a network interface and attach it to the EC2 instance that already has one network interface
+B. Explain why would anyone use two network interfaces