By default the overcloud-full image built as part of a TripleO deployment does not have a root password set. Sometimes it can be useful to set one, particularly if you're having network setup trouble that prevents you from ssh'ing into the instance after it's deployed. One simple way to do this is as follows:
sudo yum install -y libguestfs-tools
virt-customize -a overcloud-full.qcow2 --root-password password:password
. stackrc
openstack overcloud image upload --update-existing
This will install the necessary tools, set the root password, then upload it to the undercloud Glance. There are no doubt other ways to handle setting a password, but this one is pretty simple and doesn't require rebuilding the image from scratch.
Note that to set a different password, you change the "password" after the :. So the virt-customize call would look like:
virt-customize -a overcloud-full.qcow2 --root-password password:some-other-password
