Avoid removing entries for timers that didn't stop

It seems bad to remove timer objects for timers that
didn't actually stop, so instead of removing all the
timers (including the ones that had issues) only
remove the ones that didn't have issues.

Change-Id: I1dcb589f7f026ad8eaa9145a61c7c2c386ee438e

oslo_service/threadgroup.py

@@ -116,12 +116,19 @@ class ThreadGroup(object):
                 LOG.exception(_LE('Error stopping thread.'))
 
     def stop_timers(self):
+        stopped_timers = []
         for x in self.timers:
             try:
                 x.stop()
             except Exception:
                 LOG.exception(_LE('Error stopping timer.'))
-        self.timers = []
+            else:
+                stopped_timers.append(x)
+        for x in stopped_timers:
+            try:
+                self.timers.remove(x)
+            except ValueError:
+                pass
 
     def stop(self, graceful=False):
         """stop function has the option of graceful=True/False.