Consolidated URLs with constants.

Throughout the code, tests included, we have many variables pointing to
a URL which is in turn used for some of our tests or default values. I
have created several new constants for these URLs and updated the code
accordingly.
I do prefer:

```python
from robottelo.common.constants import (
    FAKE_1_YUM_REPO, FOREMAN_PROVIDERS, OPERATING_SYSTEMS,
    SYNC_INTERVAL, TEMPLATE_TYPES)
```

Because will be less lines to get to the code.