Port old API tests for products

Make module `test_product_v2` accomplish the same goals as module
`test_product`. Remove the latter module and update the API documentation
accordingly.
@omaciel I [don't really know](https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/api/test_product_v2.py) why this test is structured as is. Review failure on my part, I guess.