BZ 1158620 is fixed, so un-skip test code

[BZ 1158620](https://bugzilla.redhat.com/show_bug.cgi?id=1158620) is fixed, which means that HTTP GET calls to `/katello/api/v2/systems` returns an `organization_id`. Un-skip some test code which relies on this attribute.

    $ nosetests tests/foreman/api/test_contentview.py -m test_subscribe_system_to_cv
    .
    ----------------------------------------------------------------------
    Ran 1 test in 73.045s

    OK