deployer dry-run pipeline

As described in &154 (comment 450082184), we need a way to safely refactor deployer without affecting ongoing deployments.

deployer will perform a dry-run deployment if CHECKMODE=true.

Implementation plan

• run a manual test dry-run deployments on gstg - https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/pipelines/350793
• run a manual test dry-run deployments ongprd-cny - https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/pipelines/376500
• run a manual test dry-run deployments on gstg,gprd-cny,gprd - https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/pipelines/378026
• create a long-lived branch named next-gen on deployer with CHECKMODE=true in gitlab-ci.yml - https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/merge_requests/340