Allow user to have a different base URL for pipeline

[tkaitchuck]

Right now everything is hard coded to /_ah/pipeline/ This is an odd string, it would be better if it were configurable.

[aozarov]

This is now supported by setting a com.google.appengine.tools.pipeline.BASE_URL system property to the same value as the url-pattern for the PipelineServlet.

For example see these web.xml and appengine-web.xml.

[soundofjw]

Also, pipe.start() on the python side accepts base_path for overriding that value.

Redacted from source:

  def start(self,
            idempotence_key='',
            queue_name='default',
            base_path='/_ah/pipeline',
            return_task=False,
            countdown=None,
            eta=None):
    """Starts a new instance of this pipeline.

    Args:
      ...
      base_path: The relative URL path to where the Pipeline API is
        mounted for access by the taskqueue API or external requests.