Rename exercises dir

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

topics/aws/exercises/elasticache/exercise.md

@@ -0,0 +1,7 @@
+## AWS ElastiCache
+
+### Objectives
+
+1. Create ElastiCache Redis
+  * Instance type should be "cache.t2.micro"
+  * Replicas should be 0