To download the minimal image of Ubuntu, please refer to the below link.

http://releases.ubuntu.com/14.04/


Download it and create a VM image of Ubuntu using Virtual box or VMware Player.

Later refer to the below link to install Open stack on this image of Ubuntu.

http://www.ubuntu.com/download/cloud/install-openstack-with-autopilot


You may refer to the below link that guide you in installing Open stack on ubuntu OS.

http://docs.openstack.org/juno/install-guide/install/apt/openstack-install-guide-apt-juno.pdf

http://www.unixarena.com/2015/08/how-to-deploy-openstack-on-ubuntu.html


Hope this helps you.