Rename exercises dir

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

topics/aws/exercises/route_53_failover/exercise.md

@@ -0,0 +1,11 @@
+## AWS Route 53 - Failover
+
+### Requirements
+
+A running EC2 web instance with an health check defined for it in Route 53
+
+### Objectives
+
+1. Create a failover record that will failover to another record if an health check isn't passing
+  1. Make sure TTL is 30
+  2. Associate the failover record with the health check you have