Add a couple of AWS exercises

Questions as well were added.

exercises/aws/security_groups.md

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+## AWS EC2 - Security Groups
+
+### Requirements 
+
+For this exercise you'll need:
+
+1. EC2 instance with web application
+2. Security group inbound rules that allow HTTP traffic
+
+### Objectives
+
+1. List the security groups you have in your account, in the region you are using
+2. Remove the HTTP inbound traffic rule
+3. Can you still access the application? What do you see/get?
+4. Add back the rule
+5. Can you access the application now?