Add support for ephemeral disk placements

[hazzik]

What feature do you want to see added?

Currently ephemeral disks only support default placement which is "Temp disk" (or ResourceDisk) by default. However, Azure supports placing ephemeral disks on OS cache disks, which are generally bigger than temp disks for smaller image tiers.

The ephemeral disk placement is set up by a DiffDiskPlacement option.

Upstream changes

No response

[timja]

Is there a reason to make this configurable or should we just move the disk placement to be on CacheDisk ?

[hazzik]

I think yes. Some images don't have cache disk available for ephemeral disk placement (DC series for example), so this would be a breaking change for people using these images.