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.
Merge pull request #1793 from Ichimonji10/nailgun-client

Drop robottelo.api.client, use nailgun.client
Added two tests for activation key copy
First test verifies that you cannot use a name already taken.
Second test verifies that subscriptions are being copied over.

fixed some linting errors