This removes an assumption in the ephemeral cloud-init that
vda should be used to hold ironic images and container bindmounts.
Cloud-init will now use the ram-backed FS if there's enough
memory, and otherwise will default to vda or sda as appropriate.
Change-Id: If2a05348df7fae99598ff76e171b0e88fc2e6f88
76f89f40b3