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.

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.

exercises/aws/solutions/placement_groups.md

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+## AWS EC2 - Placement Groups          
+
+### Objectives                      
+
+A. Create a placement group. It should be one with a low latency network. Make sure to launch an instance as part of this placement group.
+B. Create another placement group. This time high availability is a priority
+
+### Solution                        
+
+A.
+1. Go to EC2 service
+2. Click on "Placement Groups" under "Network & Security"
+3. Click on "Create placement group"
+4. Give it a name and choose the "Cluster" placement strategy because the requirement is low latency network
+5. Click on "Create group"
+6. Go to "Instances" and click on "Launch an instance". Choose any properties you would like, just make sure to check "Add instance to placement group" and choose the placement group you've created
+
+B.
+1. Go to EC2 service
+2. Click on "Placement Groups" under "Network & Security"
+3. Click on "Create placement group"
+4. Give it a name and choose the "Spread" placement strategy because the requirement is high availability as top priority
+5. Click on "Create group"