These changes update the .ssh/authorized_keys rather than simply appending
This is preferable as ssh daemon picks the first key that is present.
This fixes 2 issues where something had edited a .ssh/authorized_keys
prior to cloud-init getting at it.
a.) LP: #434076 a user prior to re-bundling
b.) LP: #833499 the hypervisor
If you want to enable ssh access for root user, the proper way to do it is
with 'disable_root: False' in cloud-config.