Fix unit test run on OS X

It turns out that `os.uname` on OS X indirectly resorts to the
`exec` call which is intentionally mocked to fail. This patch
adds another mock so that the tests pass again enabling Mac folks
to engage in ironic testing! \o/

Change-Id: Ic48aed13d9ce262cb0d06ccc1d7c79f2fd2a365a

ironic/tests/unit/api/controllers/v1/test_types.py

@@ -14,6 +14,7 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+import platform
 
 import mock
 import six
@@ -114,6 +115,7 @@ class MyRoot(wsme.WSRoot):
 
 class TestJsonPatchType(base.TestCase):
 
+    @mock.patch.object(platform, '_syscmd_uname', lambda *x: '')
     def setUp(self):
         super(TestJsonPatchType, self).setUp()
         self.app = webtest.TestApp(MyRoot(['restjson']).wsgiapp())