aws/client: Fix logging to allow it to be enabled per operation

[jasdel]

Allow logging of operation and request to be enabled per operation, not only per client or session.

	client := dynamodb.New(sess)

	var w bytes.Buffer
	resp, err := client.DescribeTableWithContext(context.TODO(), params,
		func(r *request.Request) {
			r.Config.LogLevel = aws.LogLevel(aws.LogDebugWithHTTPBody)
			r.Config.Logger = aws.LoggerFunc(func(args ...interface{}) {
				fmt.Fprintln(&w, args...)
			})
		})
	if someCondition {
		log.Printf("Condition met\n%s", w.String())
	}

[skotambkar]

need to fix failing tests