Fixing test_add_configtemplate for CLI OS.

Had to change the subcommands for adding and removing a configuration
template to an operating system:

* add-configtemplate => add-config-template
* remove-configtemplate => remove-config-template

Also renamed the action methods and places where these were being called
accordingly:

* def add_configtemplate => add_config_template
* def remove_configtemplate => remove_config_template

Failed test now passes:

```bash
$ nosetests -c robottelo.properties
tests/foreman/cli/test_os.py:TestOperatingSystem.test_add_configtemplate
@test: Add configtemplate to os ... ok

----------------------------------------------------------------------
XML: foreman-results.xml
----------------------------------------------------------------------
Ran 1 test in 84.473s

OK
```