Introduce support for persistent state in /mnt.

Since instances that have ephemeral volumes, which we plan to use for
persistent state (yes the name is confusing) start with the volume
empty, we need some support to aid in placing state files (and
configuration files we want to generate dynamically) on that partition
- we can't just move stuff there during build, or it will be mounted
over by cloud-init.

Change-Id: Iff9e329ba6217e5f7290b7ab65b1808f4a8d8f9a