Rate exceed when building a lot of stacks in paralle

When we build a sequence of stacks where many can be built in parallel, e start hitting the rate limit for querying the AWS API:
```
Process Process-9:
Traceback (most recent call last):
File "/opt/rh/python27/root/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
self.run()
File "/opt/rh/python27/root/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
self._target(*self._args, **self._kwargs)
File "/var/lib/jenkins/jobs/mongodb-opsmanager/workspace/env/lib/python2.7/site-packages/stacker/providers/aws/default.py", line 152, in tail_stack
self.tail_stack(stack, retries=retries + 1, **kwargs)
File "/var/lib/jenkins/jobs/mongodb-opsmanager/workspace/env/lib/python2.7/site-packages/stacker/providers/aws/default.py", line 146, in tail_stack
include_initial=False)
File "/var/lib/jenkins/jobs/mongodb-opsmanager/workspace/env/lib/python2.7/site-packages/stacker/providers/aws/default.py", line 196, in tail
events = self.get_events(stack_name)
File "/var/lib/jenkins/jobs/mongodb-opsmanager/workspace/env/lib/python2.7/site-packages/stacker/providers/aws/default.py", line 173, in get_events
StackName=stackname
File "/var/lib/jenkins/jobs/mongodb-opsmanager/workspace/env/lib/python2.7/site-packages/botocore/client.py", line 253, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/var/lib/jenkins/jobs/mongodb-opsmanager/workspace/env/lib/python2.7/site-packages/botocore/client.py", line 543, in _make_api_call
raise error_class(parsed_response, operation_name)
ClientError: An error occurred (Throttling) when calling the DescribeStackEvents operation (reached max retries: 4): Rate exceeded
```

This is with 1.0.0.

Seems like this should be handled more gracefully by stacker, perhaps.