Allow for other tasks to add Add root user authorized_keys

Currently the "Add root user authorized_keys" task in tasks/10_system_vetting.yml sets the content of the entire file.

This does not work with Proxmox where each host in the cluster needs to add its own authorized key to the file. I presume it could also be an issue for other services, etc.

I think this task should be changed to just ensure that the file contains the expected authorized key, rather than manage the entire file.