Openstack Image Default Username Reference
This seems like something that should exist on the internet and kind of doesn't (or should at least be more searchable). These are only valid if you obtain an official image from the respective projects repository, and may or may not be the same if you use a manual image.
|rhel_7.2_x86_64||cloud-user (snowflake much?)|
If you're still not sure, I tend to find which user Openstack wants me to use by trying to login to the instance as root, and get a message similar to: ` Please login as the user "cloud-user" rather than the user "root". Connection to 10.32.161.253 closed. `