sometimes the cached driver is not fetched

[rob-baron]

Here is the snippet from the log:

R = retval = fun(*args, **kwargs)

File "/opt/env/atmo/local/lib/python2.7/site-packages/celery/app/trace.py", line 438, in protected_call
return self.run(*args, **kwargs)
File "service/tasks/monitoring.py", line 583, in monitor_instances_for
instance_map = _get_instance_owner_map(provider, users=users)
File "service/monitoring.py", line 411, in _get_instance_owner_map
all_instances = get_cached_instances(provider=provider, identity=account_identity, force=True)
File "service/cache.py", line 108, in get_cached_instances
instances_method = cached_driver.list_instances
AttributeError: 'NoneType' object has no attribute 'list_instances'

Am checking the cached_driver object to ensure that it is not None.