Rename exercises dir

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

topics/aws/exercises/create_ami/exercise.md

@@ -0,0 +1,11 @@
+## EC2 - Create an AMI
+
+### Requirements
+
+One running EC2 instance
+
+### Objectives
+
+1. Make some changes in the operating system of your instance (create files, modify files, ...)
+2. Create an AMI image from running EC2 instance
+3. Launch a new instance using the custom AMI you've created