Xds API v2 deprecation

Xds api v2 has been "softly" deprecated in latest Envoy release. Our integration tests fail with errors like:
```
gRPC config for type.googleapis.com/envoy.api.v2.Listener rejected: The v2 xDS major version is deprecated and disabled by default. Support for v2 will be removed from Envoy at the start of Q1 2021. You may make use of v2 in Q3 2020 by setting '--bootstrap-version 2' on the CLI for a v2 bootstrap file and also enabling the runtime envoy.reloadable_features.enable_deprecated_v2_api flag. (envoy.api.v2.Listener)
[tc-okhttp-stream-439430225] INFO 🐳 [envoyproxy/envoy-alpine-dev:latest] - STDERR:
```
Short term fix would be to add `-bootstrap-version 2` to Envoy container used in tests, long term to get rid of all v2 tests/code. This blocks all incoming PRs.