Add new setting in order to control concurrency in virtualized environments.

This is a fix for an issue that I ran into when trying to use django-pipeline in Circle CI. 

Circle CI is a virtualized environment using Docker containers. django-pipeline attempts to compile assets concurrently (even if PIPELINE_ENABLED is False). Circle CI virtual hosts have 32 cores and that's exactly what the container sees, even though it only really has access to 2 of those cores. When django-pipeline attempts to use all 32 cores, it fails catastrophically with a SIGABRT.

This setting allows users to override the concurrency instead of allowing django-pipeline to use as many threads as it thinks there are cores.