Convert and delete some old API tests

Remove the following modules:

1. `test_computeresource`
2. `test_configtemplate`
3. `test_domain`
4. `test_environments`

The first three contained no working tests and no interesting test stubs. The
existing test stubs were for goals such as creating or deleting an entity. Given
that all four relevant entities are already tested in module
`test_multiple_paths`, there is no value to the stubs.

The fourth module contains tests for creating environments with valid and
invalid names. Port these tests to module `test_environment_v2`.

Update the API documentation to reflect these changes.
Merge pull request #1381 from Ichimonji10/convert-tests

Convert and delete some old API tests
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.