'Returns the stack policy for a specified stack. If a stack
doesn\'t have a policy, a null value is returned.
:type stack_name_or_id: string
:param stack_name_or_id: The name or stack ID that is associated with
the stack whose policy you want to get.
:rtype: string
:return: The policy JSON document'

def get_stack_policy(self, stack_name_or_id):

    params = {'ContentType': 'JSON', 'StackName': stack_name_or_id}
    response = self._do_request('GetStackPolicy', params, '/', 'POST')
    return response['GetStackPolicyResponse']['GetStackPolicyResult']['StackPolicyBody']
