Add a couple of AWS exercises

Questions as well :)

exercises/aws/solutions/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
+
+### Solution
+
+1. Go to EC2 service -> Instances
+  1. Write down current public IP address
+  2. Click on "Instance state" -> Stop instance -> Stop
+  3. Click on "Instance state" -> Start Instance
+  4. Yes, the public IP address has changed
+2. Let's use an Elastic IP address
+  1. In EC2 service, under "Network & Security" click on "Elastic IP"
+  2. Click on the "Allocate elastic IP address" button
+  3. Make sure you select "Amazon's pool of IPv4 addresses" and click on "Allocate"
+  4. Click on "Actions" and then "Associate Elastic IP address"
+    1. Select "instance", choose your instance and provide its private IP address
+    2. Click on "Associate"
+  5. Now, if we go back to the instance page, we can see it is using the Elastic IP address as its public IP
+
+Note: to remove it, use "disassociate" option and don't forget to also release it so you won't be billed.