Decouple release manifests + standardize release script

[askmeegs]

Fixes #375 + is the first step towards kustomize support: #536

Change summary:

• Decouples release/kubernetes-manifests.yaml into separate files per service. This will support kustomize use cases (patches on individual services) including one I'm about to contribute for non gcp environments (fixing RecommendationService crashes under load-testing - "Request had insufficient authentication scopes." #359)
• Standardizes the hack release script with what's currently in bank of anthos -- https://github.com/GoogleCloudPlatform/bank-of-anthos/tree/master/release to take dev-kubernetes-manifests/ and write to kubernetes-manifests/.
• Note - keeping the release manifests in release/ so as to not break existing tutorials. The release pipeline generates both kubernetes manifests (separate files) and release/kubernetes-manifests (1 compiled file)
• Updates push-and-deploy-latest GH Actions workflow to push the latest images to the google-samples project, not onlineboutique-ci. this way the release and latest images live in the same GCR repo

TODO

• fix substitute command (s/latest/release-tag)
• handle the redis exception (do not substitute tag, use redis-alpine for both dev and release)

[github-actions]

🚲 PR staged at http://35.237.255.144

[github-actions]

🚲 PR staged at http://35.237.255.144

[github-actions]

🚲 PR staged at http://35.237.255.144

[askmeegs]

Closing this in favor of a series of smaller PRs, in line with the kustomize + sample app PRD.