Implement the fallback for service.name

[fbogsany]

open-telemetry/opentelemetry-specification#1269 defines a required fallback for service.name of "unknown_service:#{process.executable.name}" or "unknown_service" if the executable name is not available.

We do not presently provide a default in the Configurator. The Jaeger exporter defaults to 'unknown'.

We should implement the specified required fallback in Configurator and remove the default in the Jaeger exporter.