[cloud-init] Harmonize the default_user (node-admin for all)

The file /etc/cloud/cloud.cfg provides a default user creates during the cloud-init process. By default, when we use the cloud-init package provided by the OS vendor, this default_user is set regarding the name of the OS vendor name (ubuntu user for Ubuntu OS, suse for Opensuse).

This mechanism is overridden for the flavor hardened of Ubuntu, by changing the default ubuntu user by node-admin.

At some point and due the fact we manage multiple OS vendors, I suggest to override the name of the default_user of all flavors and use the same as we already set for the hardened flavor of Ubuntu, node-admin.