Deploy Gitter webapp with Ubutnu 16.04 Xenial AMIs

The xenial changes that have been in development and testing for a few months are now merged to master https://gitlab.com/gitlab-com/gl-infra/gitter-infrastructure/-/merge_requests/205#c9d8c45c4d23a916ace6c06c44a439a70003e432

Deployment process

• build a webapp xenial AMI
• Change the production ASG Launch Configuration to use the new AMI - commit tfstate changes separately for easier rollback
• scale up to 9 instances and test that webapp-09 is working well
• scale down back to 8 and replace 4 instances and leave them overnight
• replace remaining 4 instances
• perform staging deployment
• perform production deployment

Rollback process

• git revert a8b9c08b reverts https://gitlab.com/gitlab-com/gl-infra/gitter-infrastructure/-/commit/a8b9c08b backporting xenial branch to master
• git revert af007868 reverts https://gitlab.com/gitlab-com/gl-infra/gitter-infrastructure/-/commit/af007868 - commit with terraform change that updated to xenial AMI
• terraform apply to update the Launch Configuration back to trusty AMI
• kill the new xenial instances so the ASG replaces them with trusty instances