Propose: let PD consider different disk for balance

[siddontang]

although we advise users to deploy all TiKV on SSD, sometimes, the users still use HDD or even different disks.

If the users deploy TiKVs on different disks, this may cause a problem (Wooden Barrel Theory), because PD will balance the traffic fairly, and the TiKVs on the HDD will suffer the same load with other TiKVs on the SSD and provide a high latency service, which can cause the total performance reduction.

Although we can use label or set weight, etc. manually to avoid this, it is still better to let PD know the system information for better balance. The TiKV can report its disk type (HDD or SSD) to the PD, and PD will prefer balancing Hot Leaders on the SSD over HDD.

/cc @disksing @nolouch