Make "env" subcommands idempotent

[SoManyHs]

Currently, env ls depends on a call to Systems Manager to list the environments. However, if the associated CFN stack for that environment has been deleted, env ls will still succeed.

This may also affect issues with adding environments in a pipeline, e.g.

Related:

• env init workflow can't write env SSM entry if user cancels during active deployment #301
• App init idempotency: Make "app init" idempotent #552

[SoManyHs]

Similarly, env delete will fail if the CFN stack for the environment was deleted outside of this tool:

~/aws-reinvent-2019-trivia-backend(master)$ ecs-preview env delete

? Which environment would you like to delete? prod

? Which named profile should we use to delete prod? default

? Are you sure you want to delete environment prod from project backend? Yes
✘ Failed to delete environment prod from project backend: failed to find a stack named backend-prod.

~/aws-reinvent-2019-trivia-backend(master)$ ecs-preview env ls
prod
test

[iamhopaul123]

Close this one for now since:

1. we probably don't need to do it for env ls
2. for env delete I tried to delete the CFN stack then run env delete, it was successful.
3. merged in a PR addressing env init problem.

Feel free to reopen it if any idempotency issue concerning env happens!