Rename exercises dir

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

topics/aws/exercises/password_policy_and_mfa/exercise.md

@@ -0,0 +1,12 @@
+## AWS IAM - Password Policy & MFA
+
+Note: DON'T perform this exercise unless you understand what you are doing and what is the outcome of applying these changes to your account
+
+### Objectives
+
+1. Create password policy with the following settings:
+  1. At least minimum 8 characters
+  2. At least one number
+  3. Prevent password reuse
+
+2. Then enable MFA for the account.