Use a timeutils.StopWatch for cancel timing

Instead of using time.time and such directly the
oslo utils timeutils code provides a handy timer that
itself uses monotonic time (so it won't be affected
by time shifts) so prefer to use that for the timer like
code that exists in this new cancel code.

Change-Id: Ie41f8e90cf1cb3f62fd0def00b0bc7068acd2c40

oslo_service/threadgroup.py

@@ -11,16 +11,16 @@
 #    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 logging
 import threading
-import time
 
 import eventlet
 from eventlet import greenpool
 
 from oslo_service._i18n import _LE
 from oslo_service import loopingcall
-
+from oslo_utils import timeutils
 
 LOG = logging.getLogger(__name__)
 
@@ -178,10 +178,10 @@ class ThreadGroup(object):
         if timeout is None:
             return
         wait_time = kwargs.get('wait_time', 1)
-        start = time.time()
+        watch = timeutils.StopWatch(duration=timeout)
+        watch.start()
         while self._any_threads_alive():
-            run_time = time.time() - start
-            if run_time < timeout:
+            if not watch.expired():
                 eventlet.sleep(wait_time)
                 continue
             LOG.debug("Cancel timeout reached, stopping threads.")