Replace oslo_utils.timeutils.isotime

Function 'oslo_utils.timeutils.isotime()' is deprecated in version '1.6'
and will be removed in a future version. We use
datetime.datetime.isoformat() instead. For more informations:
http://docs.openstack.org/developer/oslo.utils/api/timeutils.html#oslo_utils.timeutils.isotime

Change-Id: I17384c369fdc7f86b37fd62370d800ed2463adbe
Closes-Bug: #1514331

watcher/objects/utils.py

@@ -128,7 +128,7 @@ def dt_serializer(name):
     """Return a datetime serializer for a named attribute."""
     def serializer(self, name=name):
         if getattr(self, name) is not None:
-            return timeutils.isotime(getattr(self, name))
+            return datetime.datetime.isoformat(getattr(self, name))
         else:
             return None
     return serializer