Add Capability support to AWS CloudFormation plugin

[rstml]

When creating stacks involving IAM, it's required to include IAM capability in the request. Request will look like following, note Capabilities.member.1=CAPABILITY_IAM :

https://cloudformation.us-east-1.amazonaws.com/
  ?Action=CreateStack
  &StackName=MyStack
  &TemplateBody=[Template Document]
  &Capabilities.member.1=CAPABILITY_IAM
  &...

It would be good to have configuration parameter capabilities:

<configuration>
   ...
   <capabilities>CAPABILITY_IAM</capabilities>
   ...
</configuration>

See:
http://docs.aws.amazon.com/AWSCloudFormation/latest/APIReference/API_CreateStack.html

PS: Same applies to NotificationARNs.member.N, but it's not used in my case.