Geo: Rename field of /api/v4/geo/proxy_git_push_ssh/proxy

This came out of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30687. Right now the output field is used in Geo API response for /api/v4/geo/proxy_git_push_ssh/proxy. We should rename it something that's specific to Geo so that we can add it to filter_parameters with confidence.