This method is not supported (at least that I know of), but is the only way I can find to truly proxy all outbound requests.
Use the systemd utility to open the service override file
systemctl edit glance.serviceAdd the following configuration lines - modify to fit your setup
[Service]
Environment="http_proxy=http://forward-proxy.int.example.com:3128"
Environment="https_proxy=http://forward-proxy.int.example.com:3128"
Environment="no_proxy=localhost,127.0.0.1,.int.example.com"Save/exit, then restart glance
systemctl restart glance.service