使用 cloud-init 模板

https://cloud.centos.org/

https://serverfault.com/questions/770674/centos-7-generic-cloud-login

I have an OpenStack cloud and added the CentOS-7-x86_64-GenericCloud-1503 image and it boots just fine, but I do not know how to login. What is the root password? How can I set it? Is there some other account I need to use?

If you downladed the GenericCloud image from cloud.centos.org, it does not have any users created. You can change the root password using virt-customize from libguestfs-tools package:

# virt-customize -a image_name.qcow2 --root-password password:your.great.password

Just keep in mid that this is not secure. After you login as root, you should create a sudo user, set up key-based authentication for ssh, disable root login, etc.