Comment 0 for bug 1028501

cloud-inits' mirror selection does not include any logic to account for arch.
the arm arch are not available on ec2 mirrors nor on *.archive.ubuntu.com .

The best work around at the moment is to disable mirror selection via:
 apt_preserve_sources_list: True

For juju, that can be done by modifying juju/providers/common/utils.py to contain:
 cloud_config = {
     "apt-update": True,
     "apt-upgrade": True,
     "apt_preserve_sources_list": True,
     "ssh_authorized_keys": authorized_keys,
     "packages": [],
     "output": {"all": "| tee -a /var/log/cloud-init-output.log"}}