Set path for delivery-cmd, env vars only if there
Set path for delivery-cmd, env vars only if there

This change does three things:
1. We are not passing a path to delivery-cli this means we are
   relying on the node to set it in some other way. By setting
   it to the same thing Chefdk uses we ensure we have access to
   all the things. (Like bundler for rspec...)
2. We no only set http_proxy, https_proxy, and no_proxy in the env
   of delivery-cmd if there are values. This fixes a bug where if
   you don't set it to something we set empty string that results
   in a URI.parse error deep in chef core.
   See: https://github.com/chef/chef/pull/3342
3. Honor http_proxy, https_proxy, and no_proxy set in env. If the
   env delivery-cmd is run in has env vars set we should probably
   honor them. Uses following precidence: Chef::Config, Upcase env,
   Lowcase env.

chef-cookbooks/delivery_build,chef-cookbooks/delivery_build,chef-cookbooks/delivery_build