All cloud images by default have no root password set for security reason and instead inject a user-defined SSH key. In case you need to setup a root password for console login it is easy to do it by using cloud-init and the customization script available during Instance creation.
In the “Launch Instance” wizard of Horizon dashboard, go to the “Configuration” Section and enter the following code to the “Customization Script” text box.
#cloud-config chpasswd: list: | root:o~s@grnet
After instance creation you can go to the console and login with the user root and password o~s@grnet