Add a couple of AWS exercises and questions

MORE! :)

exercises/aws/mysql_db.md

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+## 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