Improve `bst init`, including 2.x upgrade assistance

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1296)
In GitLab by [[Gitlab user @devcurmudgeon]](https://gitlab.com/devcurmudgeon) on Apr 28, 2020, 09:52

Currently the story for helping 1.x projects to migrate is not well-defined. Based on IRC discussion `bst init` could be extended to provide some help for users as follows:

- running `bst init` where no project.conf exists should create project.conf as usual, but include all default configuration with comments describing what each config does

- running `bst init` where there is an existing project.conf should attempt to upgrade the project.conf file to match current bst versioning, include any missing configuration, and report any known incompatibilities ideally with guiadance on how to fix

Start at the `bst init` command and trace how it creates or reads `project.conf`. Review the existing configuration defaults and versioning behavior; done means new projects receive commented defaults, existing projects can be upgraded, and known incompatibilities are reported with guidance.

Written by the indexing model from the issue text.