LXD REST API connection goes via proxy

When MAAS talks to LXD it does so over a HTTP(S) connection using pylxd.

pylxd in turn uses requests to talk HTTP, which will respect os.environ["http_proxy"] and os.environ["https_proxy"] accordingly. If there are scenarios where those environment variables are set for the process that executes the pylxd commands, it will cause MAAS to try and reach the LXD endpoint via the provided proxy/ies.

This can be seen e.g. in systemtests

- Marking node failed: Power on for the node failed: Failed talking to node's BMC: Pod mg8atc: Failed to connect to the LXD REST API: HTTPSConnectionPool(host='1.1.1.1', port=8443): Max retries exceeded with url: /1.0?project=default (Caused by ProxyError('Cannot connect to proxy.', OSError('Tunnel connection failed: 403 Forbidden')))