For a server booted from a volume, nova API does not store an image_id
and instead returns an empty string. Currently, openstackclient
similarly shows an empty string for Image Name and Image ID for servers
booted from volumes.
To aid CLI users in understanding the meaning of no image_id, we can
display the string "N/A (booted from volume)" in the image field if the
server was booted from a volume.
Change-Id: I9c62cf6fe23b2e934dcbf5ebbf706b2705d2e424