Go on the Ubuntu website and follow their instructions for downloading and flashing, and do not boot the Pi at this time.
Eject the sd card from the pc you used to flash, and remount it.
Go to /etc/cloud/cloud-init.cfg-d and add a a file containing this, editing your credentials in:
And thank the great tcoupin on github for writing the file. Now you can boot the Pi, and assuming you added your ssh keys in .ssh/authorized_keys, all is well.