Rename exercises dir

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

topics/aws/exercises/mysql_db/exercise.md

@@ -0,0 +1,14 @@
+## AWS Databases - MySQL DB
+
+### Objectives
+
+1. Create a MySQL database with the following properties
+  * Instance type: db.t2.micro
+  * gp2 storage
+  * Storage Auto scaling should be enabled and threshold should be set to 500 GiB
+  * Public access should be enabled
+  * Port should be set to 3306
+  * DB name: 'db'
+  * Backup retention: 10 days
+
+2. Create read replica for the database you've created