Skip unsupported API tests.

The API tests for permissions uses code which is not well understood or
supported. Skip those tests.

    $ nosetests tests/foreman/api/test_permission.py
    SSSSSSSS
    ----------------------------------------------------------------------
    Ran 8 tests in 0.002s

    OK (SKIP=8)
Merge pull request #1688 from Ichimonji10/skip-test

Skip unsupported API tests.