Make Entity.path overrides more consistent

The base `Entity` class provides a `path` method, and numerous subclasses extend
this method. Make those overrides more consistent:

* Provide the `which` variable as an argument when `super` is called.
* Use definition lists in docstrings.
Merge pull request #1482 from Ichimonji10/entity-path

(API) Make Entity.path overrides more consistent
(API) Port old tests for hosts

Significantly update the definition of class `Host`: modify fields and override
the `build` method to properly create dependent entities. Enable CRUD tests for
hosts in module `test_multiple_paths`. Drop old CRUD tests for hosts.

Slightly clean up module `test_host_v2`.

Fix #1446.