Add unit tests for `nailgun.client`

Copy unit tests from Robottelo's `tests.robottelo.test_api_client` module and
make some minor adjustments to imports and paths as necessary. Unit tests pass:

    $ python -m unittest discover --start-directory tests --top-level-directory .
    ..............No handlers could be found for logger "nailgun.client"
    ................
    ----------------------------------------------------------------------
    Ran 30 tests in 0.006s

    OK

The warning above is due to a mis-configured logger, and that can be addressed
in a separate commit.