Use default settings for Config object

[humitos]

Our current Config object is using hardcoded values to define some DOCKER settings:

https://github.com/rtfd/readthedocs.org/blob/8466fa45a2fb1b813ef0272b1154e0c98fe62218/readthedocs/config/config.py#L60-L81

This should follow the pattern that we were using at

https://github.com/rtfd/readthedocs.org/blob/8466fa45a2fb1b813ef0272b1154e0c98fe62218/readthedocs/doc_builder/constants.py#L37-L44

This way, we can modify them by setting the values as a setting and also allow to override them from outside (like local_settings.py)

Related to #2140

[humitos]

The same happens at

https://github.com/rtfd/readthedocs.org/blob/8466fa45a2fb1b813ef0272b1154e0c98fe62218/readthedocs/config/config.py#L278-L279

[humitos]

Also, modifying PYTHON_SUPPORTED_VERSIONS has no effect because it then uses

https://github.com/rtfd/readthedocs.org/blob/8466fa45a2fb1b813ef0272b1154e0c98fe62218/readthedocs/config/config.py#L283-L289

and returns at line 286 where the class variable was not used.

I found this flow kind of confusing and it's hard to know where we need to touch to get a new value supported.

It would be good if we can centralize this on only one setting/variable.