http_proxy via environment

Author: cassell (cassell)

Redmine Issue: 5694, https://pulp.plan.io/issues/5694

The pulp daemon should honor the standard lower-cased proxy environment variables; specifically, `http_proxy`, `https_proxy`, and `no_proxy`. -- some people add `ftp_proxy` to the mix, but I think ftp is mostly extinct, so no harm in skipping it, IMO.

These can be configure for all daemons system-wide in /etc/systemd/system.conf (or associated conf snippet), or per-service with a systemd drop-in, such as

/etc/systemd/system/pulp.service.d/proxy.conf:
Environment=http_proxy=http://proxy.example.com:3128
Environment=https_proxy=http://proxy.example.com:3128
Environment=no_proxy=example.com

It shouldn't matter how the environment variables are set; the daemon should honor them.

(To be fair, I didn't actually test that it doesn't honor them today; just that many companies have a http proxy in the way of getting out to the internet, and it's a common stumbling block.)

No file or test is named in the issue. Start by locating the daemon's outbound HTTP configuration and existing proxy handling, then check how environment variables are read. Done means lower-case http_proxy, https_proxy, and no_proxy are honored regardless of how they are supplied, with coverage for the supported behavior.