Updated some UI modules to use api's for dependent entities

- Updated arch tests to use api to create OS
- Corrected indentation for few template tests
- Updated all usergroup tests to use fauxfactory and blocked tests with bz-id
- Updated subnet tests to create domain using api
- Updated OS tests to use apis to create partition_table, config_templates, arch, OS and media
- Updated all media tests to use media URL from Constants
- Using media URL from Constants for location tests
- Removed single value dict from test data
- Clubbed two negative create domain tests in one
- Clubbed two negative create config_group tests in one
- Clubbed two negative compute resource tests in one
Better to format like this:

```python
from robottelo.common.constants import (
    OS_TEMPLATE_DATA_FILE, INSTALL_MEDIUM_URL)
```

That will give more room to add more entries if needed and avoid too many lines.