Add PROXY_CACHE_USE_STALE env variable to configure proxy_cache_use_stale nginx parameter.

[mhbahmani]

Hi,

Feature Overview

I was using nginx-s3-gateway. There are scenarios where the S3 is returning a 404 error (due to the requested file not being present), but I prefer that the nginx-s3-gateway gives the latest version. I added http_404 to proxy_cache_use_stale, and it seems to solve my problem.
I forked the project, and I'm working on adding this feature by making proxy_cache_use_stale configurable using PROXY_CACHE_USE_STALE environment variable.