cc_mounts should ensure_dir on lower, upper and workdir params

[Ryan Harper]

Ubuntu Trusty 14.04 machine that mounts an overlayfs at /var/spool/postfix, and this works great:

fs_setup:
  - label: postfix
    device: /dev/xvde
    filesystem: ext4
mounts:
- [ /dev/xvde, /mnt/postfix-overlay ]
- [ overlayfs, /var/spool/postfix, "overlayfs", "defaults,lowerdir=/var/spool/postfix,upperdir=/mnt/postfix-overlay", "0", "2" ]
mount_default_fields: [ None, None, "auto", "defaults,nobootwait", "0", "2” ]

Upgrading the system image to Xenial 16.04 causes the overlayfs to fail to mount with the error “missing ‘workdir’”.

Turns out, between Trusty and Xenial, the “workdir” directory in overlayfs went from optional to mandatory, and now the system no longer deploys.

What needs to happen is that after the fs_setup step that creates /mnt/postfix-overlay, but before the mounts section where the overlayfs is created, the workdir and upperdir directories need to be created on the /mnt/postfix-overlay disk as /mnt/postfix-overlay/upperdir and /mnt/postfix-overlay/workdir (or something) respectively.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2916