Add a couple of AWS exercises

Questions as well were added.

exercises/aws/solutions/ec2_iam_roles.md

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+## AWS EC2 - IAM Roles
+ 
+### Requirements
+ 
+1. Running EC2 instance without any IAM roles (so you if you connect the instance and try to run AWS commands, it fails)
+2. IAM role with "IAMReadOnlyAccess" policy
+ 
+### Objectives
+
+1. Attach a role (and if such role doesn't exists, create it) with "IAMReadOnlyAccess" policy to the EC2 instance
+2. Verify you can run AWS commands in the instance
+ 
+### Solution
+
+#### Console
+
+1. Go to EC2 service
+2. Click on the instance to which you would like to attach the IAM role
+3. Click on "Actions" -> "Security" -> "Modify IAM Role" 
+4. Choose the IAM role with "IAMReadOnlyAccess" policy and click on "Save"
+5. Running AWS commands now in the instance should work fine (e.g. `aws iam list-users`)