Rails-based DAST variable single source of truth

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179293



Merged-by: Miranda Fluharty <mfluharty@gitlab.com>
Approved-by: Arpit Gogia <12347103-arpitgogia@users.noreply.gitlab.com>
Approved-by: Dheeraj Joshi <djoshi@gitlab.com>
Approved-by: David Dieulivol <ddieulivol@gitlab.com>
Approved-by: Brett Walker <bwalker@gitlab.com>
Reviewed-by: Miranda Fluharty <mfluharty@gitlab.com>
Reviewed-by: Dheeraj Joshi <djoshi@gitlab.com>
Reviewed-by: Arpit Gogia <12347103-arpitgogia@users.noreply.gitlab.com>