Use python abc in StressAction class

Fix pylint error about NotImplemented raised instead of
NotImplementedError.

Use the abc class instead of NotImplementedError.

Partial-Bug: #1346797

Change-Id: Ia9a75a53f68c28594654a07146a2a30c1ae3c412
Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
diff --git a/tempest/stress/stressaction.py b/tempest/stress/stressaction.py
index f6770ab..286e022 100644
--- a/tempest/stress/stressaction.py
+++ b/tempest/stress/stressaction.py
@@ -12,12 +12,16 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import abc
 import signal
 import sys
 
+import six
+
 from tempest.openstack.common import log as logging
 
 
+@six.add_metaclass(abc.ABCMeta)
 class StressAction(object):
 
     def __init__(self, manager, max_runs=None, stop_on_error=False):
@@ -83,6 +87,7 @@
                     self.tearDown()
                     sys.exit(1)
 
+    @abc.abstractmethod
     def run(self):
         """This method is where the stress test code runs."""
-        raise NotImplemented()
+        return