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.
Much better :)