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OpenStack doesn’t interfere with the software that runs on the VM. You can set meta data, which is one or more key-value pairs, on an instance. You can provide an SSH key to the instance, or a user data script. All these things, however, don’t change the software that runs on the instance or the data on the instance.

Rather, the instance must process this information. For example, SSH keys and user data are usually processed with cloud-init. Even though you can “inject” files, this term is misleading, and in reality the instance pulls those files; they are not forcibly added to the instance from outside.

In short, if each instance needs to do the same thing, you need to either build some custom code into the image, or provide data to cloud-init (which is included in practically all cloud images). Which brings us back to user-data.

I suppose your problem would be solved if you could define default user-data, which would be added to an instance even without the —user-data option. AFAIK this is not possible.