updated role tests to use factory implementation and made them data-driven

- All existing tests are updated to use factory implementation and updated them to data-driven
- Added few negative tests
- Currently role creation with utf8/latin1/html char is not working due to bz https://bugzilla.redhat.com/show_bug.cgi?id=1112657
so blocked related data strings with comments
Merge pull request #1389 from sghai/role-tests

updated role tests to use factory implementation and made them data-driven
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.