'Returns dict with specified output key, value and description.
:param cntx: RPC context.
:param stack_identity: Name of the stack you want to see.
:param output_key: key of desired stack output.
:return: dict with output key, value and description in defined format.'

@context.request_context
def show_output(self, cntx, stack_identity, output_key):

    s = self._get_stack(cntx, stack_identity)
    stack = parser.Stack.load(cntx, stack=s)
    outputs = stack.outputs
    if (output_key not in outputs):
        raise exception.NotFound((_('Specified    output    key    %s    not    found.') % output_key))
    stack._update_all_resource_data(for_resources=False, for_outputs={output_key})
    return api.format_stack_output(outputs[output_key])
