Add CAPABILITY_NAMED_IAM to cloudformation capabilities

While from the documentation[1] one would assume that replacing
CAPABILITY_IAM with CAPABILITY_NAMED_IAM; this as empirically been shown
to not be the case.

1: "If you have IAM resources, you can specify either capability. If you
have IAM resources with custom names, you must specify
CAPABILITY_NAMED_IAM."
http://docs.aws.amazon.com/AWSCloudFormation/latest/APIReference/API_CreateStack.html

cloud/amazon/cloudformation.py

@@ -319,7 +319,7 @@ def main():
                              stack_policy_body=stack_policy_body,
                              template_url=template_url,
                              disable_rollback=disable_rollback,
-                             capabilities=['CAPABILITY_IAM'],
+                             capabilities=['CAPABILITY_IAM', 'CAPABILITY_NAMED_IAM'],
                              **kwargs)
             operation = 'CREATE'
         except Exception as err:
@@ -342,7 +342,7 @@ def main():
                              stack_policy_body=stack_policy_body,
                              disable_rollback=disable_rollback,
                              template_url=template_url,
-                             capabilities=['CAPABILITY_IAM'])
+                             capabilities=['CAPABILITY_IAM', 'CAPABILITY_NAMED_IAM'])
             operation = 'UPDATE'
         except Exception as err:
             error_msg = boto_exception(err)