Add new instance types to constants
Add new instance types to constants

For EC2 see
- https://aws.amazon.com/about-aws/whats-new/2020/07/announcing-new-amazon-ec2-instances-powered-aws-graviton2-processors/
- https://aws.amazon.com/ec2/instance-types/

For RDS see
- https://aws.amazon.com/about-aws/whats-new/2020/07/amazon-rds-mysql-mariadb-supports-additional-sizes/

cloudtools/troposphere,cloudtools/troposphere