allow to enable / disable stage with env var

as we now have a dev environment, stage could be optional.

gitlab's own ci template specifies an env var to enable staging: https://gitlab.com/gitlab-org/gitlab-ce/raw/v11.9.1/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml

we could use that as inspiration and add a STAGING_ENABLED as well which would default to false.

(would be a breaking change)