Make CNG release similar to omnibus-gitlab

Publishing CNG images is currently done in the tag pipeline of Charts, during which they are pulled from dev registry and pushed to com registry. However, since Charts are released for non-RC versions only, this means CNG images are also released for non-RC versions only. This, however means, test pipeline for stable branches of Charts fail because the images aren't available.

This MR moves the CNG publishing to CNG pipeline, as a manual job, similar to what we do with packages in omnibus-gitlab. Then, as part of the publish rake task, we play this manual job.

In Charts pipeline, we will add a job to wait for these images to appear in gitlab.com registry before continuing with the pipeline.

Related gitlab-org/build/CNG!235 (merged)

Changes made in this MR

1. Common tasks to "publish" an artifact (i.e, play manual jobs) is now abstracted to a BasePublishService class.
2. omnibus-gitlab and CNG publishing services extend this class, and alter the project and play_stages.
3. Modify specs and fixtures to reflect the new location of the publishing code.

changed milestone to %12.1

added backstage [DEPRECATED] + 1 deleted label

added 22 commits

• 95559d3b...d16fb275 - 21 commits from branch master
• a4e14b8a - Add code to play manual job in CNG project

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mentioned in merge request gitlab-org/build/CNG!235 (merged)

added 10 commits

• a4e14b8a...dac4f7b9 - 8 commits from branch master
• d8d758e6 - Add code to play manual job in CNG project
• e00666dc - Split off publish service and make it generic

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changed the description

added 7 commits

• e00666dc...a6eb4c8d - 5 commits from branch master
• 63ed6e6b - Split off publish service and make it generic
• 4965fe04 - Add code to play CNG jobs as part of publish

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@rspeicher This will hopefully be the final MR to get CNG releases (Charts release may need another MR) have a release workflow similar to omnibus-gitlab. MR description contains a summary of what I did.

/cc @twk3

unmarked as a Work In Progress

marked as a Work In Progress

I think this is ready for a review. Set it as WIP so that it won't be merged before the dependent ones - gitlab-org/build/CNG!235 (merged) and https://gitlab.com/charts/gitlab/merge_requests/825

assigned to @rspeicher and unassigned @balasankarc

added 12 commits

• 4965fe04...c4bad8f2 - 10 commits from branch master
• 67149141 - Split off publish service and make it generic
• 9edcb911 - Add code to play CNG jobs as part of publish

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added 8 commits

• 9edcb911...dceb5bfd - 6 commits from branch master
• 21641959 - Split off publish service and make it generic
• fdd84408 - Add code to play CNG jobs as part of publish

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unmarked as a Work In Progress

@rspeicher The dependent MRs have now been merged. So, I have un-WIP'd this MR. Could you take a look?