Handle and produce leading 0 in WEB_CONCURRENCY (#355)

The Node buildpack now exports a leading zero in its numbers. This lets us detect that it (or another buildpack) set the value, and overwrite it accordingly.

Fixes the issue where adding the node buildpack to a Python app would cause only one gunicorn worker to be spawned for a 1X dyno, and not two.

We also need to again produce leading zeroes in the value, so that e.g. the PHP buildpack can do the same on boot.