Make baremetal support metadata for ephemeral block-device-mapping

Make baremetal driver set instance['default_ephemeral_device'] to
/dev/sda1 since we are using the same device for root(sda3) and
ephemeral(sda1)

Change-Id: Ib2076198eacb0ea61706043b42d673c35d26f5d1
Partial-Bug: #1239524
Merge "Make baremetal support metadata for ephemeral block-device-mapping"