Drop robottelo.api.client, use nailgun.client

NailGun contains a near verbatim copy of the code in module
`robottelo.api.client`. Drop this module and reference module `nailgun.client`
instead. API tests appear to function correctly:

    $ make test-robottelo
    […]
    Ran 750 tests in 2416.066s

    FAILED (SKIP=89, errors=1, failures=2)

It is hoped that other references to `robottelo.api.client` have also been fixed.
Because this I like to create a block of import only like:

```python
import abc
import dfg

from xyz import abc
from hji import jkl
```

And then I can easily sort the import only block and from ... import block.