Fixing ephemeral disk creation.

The patch addresses 2 scenarios, which occur when
CONF.default_ephemeral_format was provided:

1. Creation of ephemeral block devices would fail,
   due to permission issues, during an attempt to format it without sudo,
   in _create_local.
2. The device/image would be formatted twice, while the final
   filesystem will differ from the one provided in
   CONF.default_ephemeral_format.

This patch will format only once, relying on the changed
disk.mkfs command, that takes CONF.default_ephemeral_format
into consideration.
It will also run the command as root only for block devices,
according to the disk_image is_block_dev property.

Closes-Bug: #1204424
Change-Id: I2bf7abc982e20eaaf7bca7cd798b4134b76d10a8
Merge "Fixing ephemeral disk creation."