ReleaseEngineering/How To/Add New AWS Subnets

From 1165432 bug 1165432 and notes here https://github.com/mozilla/build-cloud-tools/pull/72

• Add subnets to https://github.com/mozilla/build-cloud-tools/blob/master/configs/subnets.yml
• run `scripts/aws_manage_subnets.py configs/subnets.yml`
• Open bugs with netops to include new flows
• Update firewall tests to include new subnets https://github.com/mozilla/build-fwunit and ensure tests pass after netops has made the changes
• Update https://github.com/mozilla/build-cloud-tools/blob/master/configs/tst-linux64 and others to include subnets created in previous step