When deploying an Ubuntu/Debian host using the image-based deployment the root password is not set/changed. The mechanism requires the password to be set in the finish template (Preseed default finish). This can be achieved adding following lines:

<% if @host.provision_method == 'image' && root_pass.present? -%>
# Install the root password
echo 'root:<%= root_pass -%>' | /usr/sbin/chpasswd -e
<% end -%>