0.6.0 Update `atlantis`. Update VPC and subnets modules for `atlantis`

what

• Update atlantis
• Update VPC and subnets modules for atlantis

why

• Use the latest atlantis server version 0.8.0
• Allow users to choose between NAT Gateways or NAT Instances to be deployed into the public subnets to allow the servers in the private subnets to access the Internet
• In many cases, NAT Instances are cheaper than NAT Gateways, and for some use-cases (e.g. testing/demo infrastructure) are more appropriate to use (e.g. save on cost)

references

• https://github.com/cloudposse/atlantis/releases/tag/0.8.0
• cloudposse/terraform-aws-dynamic-subnets#51
• cloudposse/terraform-aws-components#180