Rename exercises dir

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

topics/aws/exercises/health_checks/exercise.md

@@ -0,0 +1,14 @@
+## AWS Route 53 - Health Checks
+
+## Requirements
+
+3 web instances in different AZs.
+
+## Objectives 
+
+1. For each instance create a health checks with the following properties:
+  1. Name it after the AZ where the instance resides
+  2. Failure threshold should be 5
+
+2. Edit the security group of one of your instances and remove HTTP rules.
+  1. Did it change the status of the health check?