test with container_metadata_discovery.json file from apm.git

There's now a cross agent spec for cgroup parsing, added [here](https://github.com/elastic/apm/pull/686).

We should consider using this file in our test suite. Problems to solve with that

1. How to sync spec to the [http client](https://github.com/elastic/apm-nodejs-http-client/) repository
2. How to deal with discrepancies between our established container parsing and the rules in this cross agent spec (the `testUbuntuCGroup` fixture yields different results