Auto DevOps use yaml map merging instead of shared before_script to allow overriding

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30693#note_192132596 (thanks @squarebracket)

• use yaml map merging to add the deploy_helpers to the jobs' script tags instead of using before_script
• add CI variables for the image repo/tag that's used in each stage

since the deploy jobs can't easily be tweaked without overriding wholesale. This would still require always manually overriding the image tag, but at least it would make things possible.