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.
Merge pull request #1382 from Ichimonji10/test-product-v2

Port old API tests for products
Add API tests for subnets

Make it possible to create, read and delete subnets using the `Subnet` class in
module `robottelo.entities`. Do this by adding several mixins to that class.
Verify that it is possible to create, read, update and delete subnets by adding
them to module `test_multiple_paths`.

Remove module `test_subnet`. It contains nothing but test stubs.