Greetings,
When you want to post, please just send a new email to the
openstack-discuss@lists.openstack.org mailing list. Replying to, and including the regular digest doesn't help other subscribers to the mailing list unless you're replying to a specific email in the digest.
As to your question, you have two options. The first is to edit the disk image using something like guestfish[0], or some other disk image tool like diskimage-builder's devuser element[1] for injecting a static credential.
The second, and what should be the default option option is to deploy your instance with a keypair to be utilized, which then would relate to a private key you have locally. Once the instance has booted, a tool like cloud-init which is present in most "cloud images", then configures the key you supplied for a user such as "cloud-user", but may be specific based upon the cloud image you are using.
-Julia