Improve consistency across platforms

[ehmatthes]

All three platforms have been refactored. Improvements in some workflows have not been ported to other platforms. This is mostly targeted at a smooth transition to a plugin-based model. Direct follow up #275.

• [ ] Test all three platforms on macOS and Windows, using configuration-only and automate-all approaches.
• [ ] Update CLI messages.
• [ ] Review documentation.
• [x] Use Parking Lot to close 5-10 oldest issues. (Closed all stale issues; all open issues fit on one page.)
• [ ] Look at what's done across the different platform deploy scripts. Identify any utility functions that can be moved to sd_utils. Move them there, and make sure all work in the three deployer scripts is unique to that platform.
  • [ ] Use modify_settings().
• [ ] Consider writing quick unit tests for each new utility function.
• [ ] Improve consistency across platforms.
  • [ ] Specify a preferred order for tasks, in the draft plugin contract.
  • [ ] Ensure that all three deployers follow that order.
  • [ ] This should be a matter of looking at the deploy() method.
    • [ ] Note architecture recommendations somewhere appropriate (not a new issue).
    • [ ] Clarify suggestion for an unclean Git status.
    • [ ] Support multiple deployments on Platform.sh.
    • [ ] Make sure Simplify overall approach matches workflow on all three platforms.
    • [x] Make sure Platform.sh isn't confirming automate_all twice, once in sd and once in self.

[ehmatthes]

Make Platform.sh consistent with Heroku and Fly improvements

• [x] Fix repeated call to confirm automate all.
• [ ] Make sure e2e tests pass:
  • [x] config-only
  • [ ] automate_all
• [x] Update settings approach:
  • [x] Check settings file for overwriting.
  • [x] Write settings file same as Heroku.
• [ ] Can _modify_settings() be moved to simple_deploy.py? (Do this after building plugin model.)

[ehmatthes]

Closing this

• [x] Update fly settings approach.
• [x] Update fly env vars approach.
• [x] Run e2e test for fly.
• [x] Merge, release.
• [x] Test fly pypi.
• [x] Test plsh pypi.
• [x] Make new release notes section.
• [x] Announce.
• [x] Reference this issue in main plugin issue.
• [x] Move on to making Platform.sh work for multiple projects.