OpenStack images for Ubuntu are available from cloudbase.it, including images for:
Follow the local setup instructions before starting.
Download image from the above URL, then run:
openstack image create --file xenial-server-cloudimg-amd64-disk1.img --disk-format qcow2 "Ubuntu 16.04"
openstack image create --file bionic-server-cloudimg-amd64.img --disk-format qcow2 "Ubuntu 18.04"