set default for config_root_path
set default for config_root_path

https://github.com/google/glazier/blob/master/docs/setup/config_layout.md#components implies that Glazier will look for the first `build.yaml` file under `config_server/$branch/config/build.yaml` but `config_root_path` is, by default, set to `''` so Glazier will try and fail to find `config_server/$branch/build.yaml`. 

This PR makes the code match the docs :)
google/glazier,google/glazier