simplify specific resource configs, they're redundant now

[m3ngyang]

configs of different resource are redundant now, for example

https://github.com/baidu/baiducloud-sdk-go/blob/develop/eip/client.go#L12

// Config of eip
type Config struct {
	*bce.Config
}

https://github.com/baidu/baiducloud-sdk-go/blob/develop/bos/client.go#L27

// Config of bos
type Config struct {
	*bce.Config
}

[m3ngyang]

Specific resources, such as eip, vpc, bos, etc, share the same config struct of bce, to construct their own clients. @wccsama @cwdsuzhou @hello2mao

[m3ngyang]

close by #19