Rename exercises dir

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

topics/aws/exercises/elastic_ip/exercise.md

@@ -0,0 +1,10 @@
+## AWS EC2 - Elastic IP
+
+### Requirements
+
+* An EC2 instance with public IP (not elastic IP)
+
+### Objectives
+
+1. Write down the public IP of your EC2 instance somewhere and stop & start the instance. Does the public IP address is the same? why?
+2. Handle this situation so you have the same public IP even after stopping and starting the instance