Added tests for org - add-subnet to org

Added tests for org - add-subnet to org
Merge pull request #1393 from tkolhar/test_cli

Added tests for org - add-subnet to org
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.