Add Environment v2 Builder (#298)

Build server and client for environments.

This build server will run inside environment builders and invoke environment-specific scripts for doing builds. It provides a uniform language-independent API for builds to the fission build manager.

The change also includes an implementation of a builder for Python -- this "builder" acts on requirements.txt and runs `pip install` to collect all function deps.

environments/python3/builder/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:3.5
+
+RUN apk update
+RUN apk add --no-cache python3 python3-dev build-base
+RUN pip3 install --upgrade pip
+RUN rm -r /root/.cache
+
+ADD defaultBuildCmd /usr/local/bin/build
+ADD builder /
+
+EXPOSE 8000