Support post-pending a tag on the urls used in a pipeline

[ckurz-va]

For DevOps purposes, we had added functionality to the deferred library to allow a task to be queued with additional information added to the task url (eg: normal task queues to "_ah/queue/deferred". We add a tag that queues it to the url "_ah/queue/deferred/BackendTaskOne"). This works because the deferred library routes are defined to just accept ".*" on your deferred mapping. With an application that uses a lot of deferred tasks, this makes path a very useful means to determine what deferred work is causing issues.

I would like similar functionality in the pipeline library. All calls to a pipeline result in the path "/mapreduce/pipeline/run" which makes it difficult to track on the app engine dashboard which pipeline is causing errors. The pipeline routes are defined as "/pipeline/run". I would propose they be defined as "/pipeline/run.*" which would allow for a custom tag to be post-pended on the path, making tracking easier.