Add a couple of AWS exercises

Questions as well :)

exercises/aws/solutions/elastic_network_interfaces.md

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+## AWS EC2 - Elastic Network Interfaces
+
+### Requirements
+
+* An EC2 instance with network interface
+
+### Objectives
+
+A. Create a network interface and attach it to the EC2 instance that already has one network interface
+B. Explain why would anyone use two network interfaces
+
+### Solution
+
+A.
+1. Go to EC2 service
+2. Click on "Network Interfaces" under "Network & Security"
+3. Click on "Create network interface"
+4. Provide a description
+5. Choose a subnet (one that is in the AZ as the instance)
+6. Optionally attach a security group and click on "Create network interface"
+7. Click on "Actions" -> "Attach" and choose the instance to attach it to
+8. If you go now to "Instances" page you'll see your instance has two network interfaces
+
+B.
+1. You can move the second network interface between instances. This allows us to create kind of a failover mechanism between the instances.