Rename exercises dir

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

topics/aws/exercises/security_groups/exercise.md

@@ -0,0 +1,20 @@
+## AWS EC2 - Security Groups
+
+### Requirements 
+
+For this exercise you'll need:
+
+1. EC2 instance with web application
+2. Security group inbound rules that allow HTTP traffic
+
+### Objectives
+
+1. List the security groups you have in your account, in the region you are using
+2. Remove the HTTP inbound traffic rule
+3. Can you still access the application? What do you see/get?
+4. Add back the rule
+5. Can you access the application now?
+
+## Solution
+
+Click [here to view to solution](solution.md)