Update the env var/system property for resource labels

[jkwatson]

Now that open-telemetry/opentelemetry-specification#666 has been merged, we should use the standard env var/system property for Resources. See EnvAutodetectResource.java for details.

[jkwatson]

Maybe not needed if open-telemetry/opentelemetry-specification#758 gets merged. ;)

[jkwatson]

closing, since the spec got updated to match us.