add init command to cli

[krancour]

Bookkeeper expects repos to be laid out a certain way, and this is partly dependent on what configuration management tool you select.

It would probably be helpful if the bookkeeper CLI had an init command that could help users set up repos according to bookkeeper's conventions.

[krancour]

The CLI has largely gone away because the corresponding server component has as well.

The only remnant of the CLI is the what was formerly referred to as the "thick" CLI -- e.g. the CLI that is shipped in a Docker image that guarantees compatible Git, Kustomize, Helm, and ytt binaries.

So, the bottom line is that if we want some kind of "bookkeeper init" functionality, it probably needs to be its own tool.