Problem with ConfigFactory.defaultReference, user.dir

akka-cluster-metrics defines:
```
native-library-extract-folder = ${user.dir}"/native"
```

but as reported in https://discuss.lightbend.com/t/akka-cluster-metrics-native-library-extract-folder-refers-to-user-dir/5108 it seems like `${user.dir}` is not found when using `ConfigFactory.defaultReference`

defaultReference is documented as:
```
* Obtains the default reference configuration, which is currently created
* by merging all resources "reference.conf" found on the classpath and
* overriding the result with system properties. The returned reference
* configuration will already have substitutions resolved.
```

not sure if that means that system properties are also used for variable substitutions.

If system properties are not involved in the substitutions we should probably make reference.conf in akka-cluster-metrics self contained by defining a default "." for `user.dir` in that reference.conf.

Inspect akka-cluster-metrics reference.conf and the ConfigFactory.defaultReference behavior described in its documentation. Determine whether system properties participate in substitutions such as ${user.dir}; done means the behavior is confirmed and the reference configuration or relevant coverage reflects the intended resolution.

Written by the indexing model from the issue text.