(#16635) Respect http_proxy environment and settings in pkgdmg

In 3.x, the pkgdmg provider no longer honors http_proxy_host or http_proxy_port
settings or environment variables. This takes the methods to do this from
puppet/forge/repository and puts them in puppet/util/http_proxy, and updates
repository.rb and pkgdmg.rb to use it.
Merge branch 'pull-1719'

This closes GH-1719