Provide a way for jobs to have a read/write root filesystem

[nfachan]

In the end, we're mounting one big overlayfs and specifying no upperdir to give us a read-only file system. We could specify an upperdir and then the file system would be read-write. It seems like there would be three options for what type the upper file system was:

1. tmpfs
2. ephemeral but disk backed
3. output

We could also provide a way to have this happen not just a /, but that may need to wait until we're doing fuse.