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.