Remove config struct from `ocis/pkg/init/init.go`

There is a structure representing the needed configuration variables in `ocis/pkg/init/init.go`.
We have two use-cases where we need to output the configuration variables: `ocis init` and when generating a list of all available environment variables for the docs (see `docs/helpers/configenvextractor.go` for details).
The problem is that the yaml package has no option to ignore the `omitempty`-tag when run, this would either result in a unusable `ocis.yaml` when generating the default config with `ocis init` or missing variables in the lists the `configextractor.go` generates.

We need to remove the said structure and replace it with a mechanism that can give us both solutions from the structs present in the source code.