Comment 11 for bug 1781039

I tested the package cloud-init 0.7.5-0ubuntu1.23 from trusty-proposed and confirm it works as expected.

# TEST KEYS

$ cat googlekeys
test:ssh-rsa AAAA[...]+1LRl <email address hidden>
ubuntu:ssh-rsa AAAA[...]+2LRl <email address hidden>
cloudinit:ssh-rsa AAAA[...]+3LRl test

# CREATE THE INSTANCE

$ gcloud compute instances create ubuntu1404cloudinittest --image-family ubuntu-1404-lts --image-project ubuntu-os-cloud --metadata-from-file=ssh-keys=googlekeys --metadata=block-project-ssh-keys=True --zone us-central1-a

# BEFORE THE UPDATE

$ gcloud compute --project "ubuntu-os-support" ssh --zone "us-central1-a" ubuntu1404cloudinittest

$ sudo cat /home/ubuntu/.ssh/authorized_keys
ssh-rsa AAAA[...]+fVtBKqZ fubar@famine
# Added by Google
ssh-rsa AAAA[...]+2LRl <email address hidden>

# AFTER THE UPDATE

$ sudo sh -c 'echo deb http://us-central1.gce.archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse >> /etc/apt/sources.list'

$ sudo apt update

$ sudo apt install cloud-init
[...]
Setting up cloud-init (0.7.5-0ubuntu1.23) ...
[...]

$ sudo sh -c 'cat /dev/null > /home/ubuntu/.ssh/authorized_keys'

$ sudo rm -rf /var/lib/cloud/instance*

$ sudo reboot

$ gcloud compute --project "ubuntu-os-support" ssh --zone "us-central1-a" ubuntu1404cloudinittest

$ sudo cat /home/ubuntu/.ssh/authorized_keys
ssh-rsa AAAA[...]+2LRl <email address hidden>
ssh-rsa AAAA[...]+3LRl test
# Added by Google
ssh-rsa AAAA[...]+2LRl <email address hidden>

cloud-init now works the same way as Ubuntu 16.04 does. Thanks!

Bonus test: I tested that oslogin also works well:

$ gcloud compute instances add-metadata ubuntu1404cloudinittest --metadata enable-oslogin=TRUE --zone "us-central1-a"
Updated [https://www.googleapis.com/compute/v1/projects/ubuntu-os-support/zones/us-central1-a/instances/ubuntu1404cloudinittest].

$ gcloud compute --project "ubuntu-os-support" ssh --zone "us-central1-a" ubuntu1404cloudinittest
[...]
Welcome to Ubuntu 14.04.5 LTS (GNU/Linux 4.4.0-137-generic x86_64)
[...]