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/build.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+builderDir=${GOPATH}/src/github.com/fission/fission/builder/cmd
+pushd ${builderDir}
+GOOS=linux GOARCH=386 go build -o builder .
+popd
+cp ${builderDir}/builder .
+docker build -t python-builder .
+docker tag python-builder fission/python-builder:$tag
+docker push fission/python-builder:$tag
+