Add env variables in build_environment.rb

An error occurred while running the follow command.
stemcell:build_with_local_os_image[openstack,ubuntu,trusty,go,/tmp/ubuntu_base_image_trusty.tgz]

Error dispaly:

++ echo '# THIS FILE IS GENERATED; DO NOT EDIT OR COMMIT'
    cd /mnt/stemcells/openstack/kvm/ubuntu/build/build/stages/aws_cli/assets
    rm -rf s3cli
    curl -L -o s3cli.tar.gz https://api.github.com/repos/pivotal-golang/s3cli/tarball/2c4a7f0ceef411532bb051e7ca55a490a565cf60
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- 0:02:08 --:--:-- 0curl: (7) couldn't
connect to host
rake aborted!
Merge pull request #19 from zoujin/hw-issue-38

When creating stemcell, proxy environment variable `https_proxy` wasn't kept, causing download failure.