Ensure pd leader is ready to serve when rolling update pd

[AndreMouche]

The PD leader needs to reload all regions' meta data into memory before it can serve, therefor for a big cluster, this process can take a relatively long time, such as 100 seconds or more. During rolling update, if we transfer the pd leader to a PD node that has just been started, before load-region success, pd cannot provide services. Here we suggest to wait some minutes before we try to transfer pd leader to a pd node that has just been started（restart the PD leader Node）.