BZ 1158620 is fixed, so un-skip test code

BZ 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