Add a couple of AWS questions and exercises

EBS & ELB mainly.

exercises/aws/app_load_balancer.md

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+## AWS ELB - Application Load Balancer 
+
+### Requirements                   
+
+Two EC2 instances with a simple web application that shows the web page with the string "Hey, it's a me, `<HOSTNAME>`!"
+
+### Objectives                     
+
+1. Create an application load balancer for the two instances you have, with the following properties
+  1. healthy threshold: 3
+  2. unhealthy threshold: 3
+  3. interval: 10 seconds
+2. Verify load balancer is working  (= you get reply from both instances at different times)