Cease setting the default forward policy to accept

[jsf9k]

🗣 Description

This pull request:

• Ceases setting the default forwarded/routed policy to ALLOW (or anything else)
• Updates README.md to let the user know that he or she will need to configure the forwarded/routed traffic via ufw outside of this role.

💭 Motivation and Context

For hardened systems the user may well want the default policy for forwarded/routed traffic to be DENY and create a rule to allow specific forwarded/routed traffic through.

See also cisagov/openvpn-packer@172e6600c667f1d17525ca8990a257d7473464a7.

🧪 Testing

I have successfully built a COOL OpenVPN AMI with these changes and verified that they work as intended.

🚥 Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (causes existing functionality to change)

✅ Checklist

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.