Work around broken cloud-init ephemeral disk code

Cloud-init cannot handle ephemeral disks like '/dev/sda1'. This prevents
the nova baremetal default from working properly. So if the ephemeral
disk isn't already mounted, we mount it just before we need it.

Change-Id: Ide0e5ed3eff91755aac7d8f1e9c43f723f7bf3d5
Closes-Bug: #1263294