Make ActivationKey.read_json poll upon getting 404

Make method `ActivationKey.read_json` poll the server up to five additional
times upon receiving an HTTP 404 response. As per an IRC conversation with
@thomasmckay, bugzilla bug 1127335 will not be fixed, and clients **must**
resort to polling the server to determine whether a 404 is valid or not.

Several tests in module `test_activationkey` fail, but due to unrelated reasons
(that should be investigated!).

    $ nosetests tests/foreman/api/test_activationkey.py
    InsecureRequestWarning)
    ......FFF...................
    ======================================================================
    …
    ----------------------------------------------------------------------
    Ran 28 tests in 86.375s

    FAILED (failures=3)

Fix the one remaining failure in module `test_multiple_paths`:

    $ nosetests tests/foreman/api/test_multiple_paths.py -m ActivationKey
    InsecureRequestWarning)
    .S....SS
    ----------------------------------------------------------------------
    Ran 8 tests in 38.545s

    OK (SKIP=3)

Fix several Jenkins failures. Target #1578.
Merge pull request #1618 from Ichimonji10/activation-key

Make ActivationKey.read_json poll upon getting 404