Enable IPv6 tests in CI

[mum4k]

Currently we only execute tests using the v4only mode:

https://github.com/envoyproxy/nighthawk/blob/4d831ff59d7a53c7aa0839757b075070c7ee927d/ci/do_ci.sh#L182

This can cause our IPv6 based integration tests to go stale. We should investigate what is required to enable IPv6 on Circle CI and start running with ENVOY_IP_TEST_VERSIONS=all.

Looks like Circle CI does support IPv6 for local traffic:

https://circleci.com/docs/2.0/faq/#can-i-use-ipv6-in-my-tests