…3793)

* Remove obsolete code

Now, we can access `self.config.build_image` directly.

* Move container_image selection to the init

At initialization time we have the project and we already know if the
project has the build image override so we can decide at that point
and save it as a instance attribute.

Then we can use this values from other places inside the same class.

* Save Docker Image hash in readthedocs-environment.json

The hash is used to know if the environment is obsolete and auto-wipe
it if necessary.

* Simplify the class naming

* Save the image hash in the json file

* Lint

* Remove invalid properties from YAML config in tests

* Add test for save_environment_json

* when the file is corrupted or we don't have access, we return that
  it's OBSOLETE

* when there is a new setting that we need to compare and it's not in
  the JSON file, we return OBSOLETE