Add and refactor API tests for products

Make `Product` inherit from `EntityReadMixin`. use this mixin to trim down
module `tests.foreman.api.test_product_v2`.

Test the ability to CRUD products by adding `Product` to module
`test_multiple_paths.py`. Remove a deletion-related test from module
`test_product_v2` because `test_multiple_paths` covers the same ground.
Refactor and better test Repository.path

Refactor method `Repository.path` by trimming down the amount of underlying
code. Better test the method: only one of the two paths was being tested.
Merge pull request #1304 from Ichimonji10/test-product

Add and refactor API tests for products
Merge pull request #1307 from Ichimonji10/repository-path

Refactor and better test Repository.path
Add API tests for config templates

Update the `ConfigTemplate` class definition: add mixins for creation, reading
and deletion. Thoroughly test the relevant API endpoints.