After the recent v3 cut, Envoy attempts to upgrade to v3 any unparseable v2 config. This leads to v3 related messages to config that the user intends as v2, a bad UX. There are a few options to make this explicit:

1. Allow version to be included in the filename, e.g. foo.v2.yaml, bar.v3.yaml.

2. Add a CLI flag to Envoy to imply an explicit version for bootstrap JSON/YAML (and others?)

CC @snowp