cloudtrail: Fix error on existing trail without S3 prefix (#2939)

lib/ansible/modules/extras/cloud/amazon/cloudtrail.py

@@ -180,6 +180,7 @@ def main():
     s3_bucket_name = module.params['s3_bucket_name']
     # remove trailing slash from the key prefix, really messes up the key structure.
     s3_key_prefix = module.params['s3_key_prefix'].rstrip('/')
+
     include_global_events = module.params['include_global_events']
 
     #if module.params['state'] == 'present' and 'ec2_elbs' not in module.params:
@@ -194,7 +195,7 @@ def main():
             results['view'] = cf_man.view(ct_name)
             # only update if the values have changed.
             if results['view']['S3BucketName']              != s3_bucket_name or \
-              results['view']['S3KeyPrefix']                != s3_key_prefix  or \
+              results['view'].get('S3KeyPrefix', '')      != s3_key_prefix  or \
               results['view']['IncludeGlobalServiceEvents'] != include_global_events:
                 if not module.check_mode:
                     results['update'] = cf_man.update(name=ct_name, s3_bucket_name=s3_bucket_name, s3_key_prefix=s3_key_prefix, include_global_service_events=include_global_events)