Set instance default_ephemeral_device

Duplicate the old nova-bm logic of setting the default ephemeral device
to /dev/sda1 (first partition on the disk) when an ephemeral partition
is being used. This is a short-term solution to fix the majority of cases
that depend on this value being present in the instance metadata
(e.g., cloud-init). Longer term, we should consider using something
like udev by-label names (/dev/disk/by-label/NNN) or similar.

Closes-Bug: #1324286

Change-Id: Ia85f0fd0bfa5cca0320db0e8c7f1e0e2b61b4ef6
Merge "Set instance default_ephemeral_device"