Add a couple of AWS questions

exercises/aws/health_checks.md

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+## AWS Route 53 - Health Checks
+
+## Requirements
+
+3 web instances in different AZs.
+
+## Objectives 
+
+1. For each instance create a health checks with the following properties:
+  1. Name it after the AZ where the instance resides
+  2. Failure threshold should be 5
+
+2. Edit the security group of one of your instances and remove HTTP rules.
+  1. Did it change the status of the health check?