Comment 9 for bug 1647708

I've verified this as described in the test case.

## fresh instance
$ dpkg-query --show cloud-init
0.7.8-49-g9e904bb-0ubuntu1~16.04.1

$ cat /etc/cloud/build.info
build_name: server
serial: 20161213

## /proc/mountsn does not have the /mnt mount, but /etc/fstab does
$ awk '$2 == "/mnt" { print $0 }' /proc/mounts
$ awk '$2 == "/mnt" { print $0 }' /etc/fstab
/dev/disk/cloud/azure_resource-part1 /mnt auto defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2

$ m=http://azure.archive.ubuntu.com/ubuntu/
$ echo deb $m xenial-proposed main universe |
   sudo tee /etc/apt/sources.list.d/proposed.list

$ sudo apt-get update && sudo apt-get install -qy cloud-init

$ dpkg-query --show cloud-init
cloud-init 0.7.8-49-g9e904bb-0ubuntu1~16.04.3

## Clean up for new reboot
# remove old entry in /etc/fstab
$ sudo sudo sed -i '/cloudconfig/d' /etc/fstab
$ sudo umount /mnt

## reformat to ntfs to look like first boot
$ disk=/dev/disk/cloud/azure_resource
$ echo "2048,1024000,7" | sudo sfdisk --force "$disk"
$ sudo udevadm settle
$ sudo umount /mnt
$ time sudo mkfs.ntfs --quick ${disk}-part1

## remove state
$ sudo rm -Rf /var/lib/cloud/ /var/log/cloud-init*
$ sudo reboot

## ssh back in ..
$ awk '$2 == "/mnt" { print $0 }' /proc/mounts
/dev/sdb1 /mnt ext4 rw,relatime,data=ordered 0 0
$ awk '$2 == "/mnt" { print $0 }' /etc/fstab
/dev/disk/cloud/azure_resource-part1 /mnt auto defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2

$ grep Activ /var/log/cloud-init.log
2016-12-20 18:52:41,270 - cc_mounts.py[DEBUG]: Activate mounts: PASS:mount -a
2016-12-20 18:52:41,507 - cc_mounts.py[DEBUG]: Activate mounts: PASS:systemctl daemon-reload

## for good measure, reboot again (to similate non-first boot).
$ sudo reboot

## ssh back in, look at /mnt is mounted
$ awk '$2 == "/mnt" { print $0 }' /proc/mounts
/dev/sdb1 /mnt ext4 rw,relatime,data=ordered 0 0