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

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+## 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.

topics/aws/exercises/password_policy_and_mfa/solution.md

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+## 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.
+
+### Solution
+
+Password Policy:
+
+1. Go to IAM service in AWS
+2. Click on "Account settings" under "Access management"
+3. Click on "Change password policy"
+  1. Check "Enforce minimum password length" and set it to 8 characters
+  1. Check "Require at least one number"
+  1. Check "Prevent password reuse"
+4. Click on "Save changes"
+
+MFA:
+
+1. Click on the account name
+2. Click on "My Security Credentials"
+3. Expand "Multi-factor authentication (MFA)" and click on "Activate MFA"
+4. Choose one of the devices
+5. Follow the instructions to set it up and click on "Assign MFA"