Add a couple of AWS exercises

Questions as well :)

exercises/aws/elastic_ip.md

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+## AWS EC2 - Elastic IP
+
+### Requirements
+
+* An EC2 instance with public IP (not elastic IP)
+
+### Objectives
+
+1. Write down the public IP of your EC2 instance somewhere and stop & start the instance. Does the public IP address is the same? why?
+2. Handle this situation so you have the same public IP even after stopping and starting the instance