Add a new method to get config from environment variables
and for now, we have three ways to get config and the
priority is `env-var > custom-config-file > built-in-config-file`.
Environment config setting does not work in CLI, because it will
confuse users when they get config value is `var-env` but value
in the configuration file is `var-in-file`, they may not find
the way how to change it
* Add documentation
* Add it to UPDATING.md
close: #8344
* Change `yaml_parser.py` method `to_string` to magic method
`__str__` make it more pythonic.
* Add some tests to magic method `__str__` and `__repr__`
* [python] refactor yaml file parser
* Change yaml parser package to ruamel.yaml
* Refactor configuration.py module
* And file.py to write file locally
* Add more tests on it
close: #8593
* Fix UT error
* Remove pypyaml from tests
* Fix file error when param create is False
* Fix error logic
* And tests to avoid regression
陈家名
Jiajie Zhong