Cannot set per-cloud environment variables

Our GCE cloud is configured to not allow direct Internet access, either via external IP or via NAT, and all traffic must leave via a proxy where it can be logged. However, the GCE plugin does not provide any way to set the required `http_proxy` et al environment variables on a per-cloud basis; the best available workaround would be to set them at the Jenkins global level, but then our non-GCE instances (bare metal, legacy EC2, corp Mac Minis, etc) will have the wrong proxy configured.