Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/aws/exercises/create_efs/exercise.md

@@ -0,0 +1,12 @@
+## AWS - Create EFS
+ 
+### Requirements
+
+Two EC2 instances in different availability zones
+
+### Objectives
+
+1. Create an EFS with the following properties
+  1. Set lifecycle management to 60 days
+  2. The mode should match a use case of scaling to high levels of throughput and I/O operations per second
+2. Mount the EFS in both of your EC2 instances