Add Flatcar Linux to the issue template
### Environment
* Platform: aws, azure, bare-metal, google-cloud, digital-ocean
-* OS: container-linux, fedora-atomic
-* Ref: Release version or Git SHA (reporting latest is **not** helpful)
+* OS: container-linux, flatcar-linux, or fedora-atomic
+* Release: Typhoon version or Git SHA (reporting latest is **not** helpful)
* Terraform: `terraform version` (reporting latest is **not** helpful)
* Plugins: Provider plugin versions (reporting latest is **not** helpful)
