(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.
That could be nice, especially as `search` and `update` are coming down the line at some point.

In the near future, I want to make an `orm.EntityCreateMixin` that provides methods named `create*`. For example, `create`, `create_dependents` and `create_attributes`... or something like that. This is in line with what the `orm.EntityReadMixin` does: it provides `read` and `read_json`.