Intuitive default behavior for sdk_location pipeline option

The current default value of "default" implies a Dataflow specific behavior of the artifact stager. The same stager is also used by the portable runner, which has to specify a value "container", which actually means to not stage the SDK. That should be the default behavior and the default value for the sdk_location should be None. The Dataflow runner can then specify a value such as "pypi" which conveys more closely the expected behavior.