This is an attempt to implement circus-tent#1119. For now only configs and tests
are added and some drafts in the config loader. This is still work in
progress.

- rename config in tests
- rename test case
- add setting of config option from include

- Refactor read_config method with two new local methods

  This was necessary to avoid code duplication, because include directive
  in `[watcher]` is basically the same as in `[circus]`. This allowed the reuse of `_scan` method. I also added underscores to
  arguments names, because they were shadowed.

  - `_init_config` - takes config path, has a parser option with default value
    and return parsed config from the path
  - `_get_includes` - takes config object and section name and returns a
    list of paths to read options from

- Add a required section name for watcher includes to avoid usage of a
  fake `[tmp]` section from previous commits
  according to [documentation](https://docs.python.org/3/library/configparser.html)
  sections are required and in my opinion should be used going forward

-  Expand test case to ensure included configs can be loaded from folders
   and using wildcards