New setting for debuging django-pipeline.

Normally, when DEBUG=True, PIPELINE_ENABLED defaults to False, so
we can't test the pipeline files work correctly.

However, simply by changing PIPELINE_ENABLED to True, we get 404 errors,
as Django staticfiles method gets used, and it doesn't understand
django-pipelines system of generating files when running collectstatic.

So we have to turn DEBUG=False, which will change the default
PIPELINE_ENABLED to True. However then we need to serve the static files
ourselves. This is what the DEBUG_SERVE_STATIC setting controls.

Change-Id: Ib2aa06dcb9ff96ab359b3b0d26379e6583d26864