Implement the meat for DescribeStackEvents

Right now this just adds events for the stack itself via the lifecycle
methods of the FakeStack object, but it is possible to add other kinds
of events (I left a method for that should someone need inspiration
later.)

moto/cloudformation/responses.py

@@ -281,7 +281,7 @@ DESCRIBE_STACK_EVENTS_RESPONSE = """<DescribeStackEventsResponse xmlns="http://c
     <StackEvents>
       {% for event in stack.events %}
       <member>
-        <Timestamp>{{ event.timestamp }}</Timestamp>
+        <Timestamp>{{ event.timestamp.strftime('%Y-%m-%dT%H:%M:%S.%fZ') }}</Timestamp>
         <ResourceStatus>{{ event.resource_status }}</ResourceStatus>
         <StackId>{{ event.stack_id }}</StackId>
         <EventId>{{ event.event_id }}</EventId>