GCP: GCP can attach Local SSD as SCSI disk, but gcp_instance misunderstands them as 'persistent disk'

[syuu1228]

Since our code expects Local SSD are attached as NVMe disk and persistent disks are attached as SCSI disk, the code will misunderstand SCSI attached Local SSD as 'persistent disk'.

see: https://cloud.google.com/compute/docs/disks/local-ssd

Local SSDs are available through both SCSI and NVMe interfaces.

Probably we don't have to support it since NVMe is faster, but it should documented, and/or describe it in the login prompt.

[yaronkaikov]

@syuu1228 is this relevant?