Added the possibility for Build to use NODE_ENV before defaulting

[fede]

NODE_ENV was hardcoded to 'production'. This made it impossible to set staging, integration or any other NODE_ENV.

[annez]

I completely agree with this change, builds shouldn't be hardcoded to any sort of environment because people choose to name things differently, and in most environments you will have a staging as well. Unless @jaredpalmer you suggest doing this another way for, say, a URL config map of API endpoints?

We have things like:

{
   "production": "https://foo",
   "staging": "https://bar",
   "default": "localhost"
}

These URLs are shared across both client and server, and currently we cannot use the same environment variable to generate the right builds before running this.

[jaredpalmer]

Closed in favor of #611