Fix sshd configuration with pre-existing drop-in files

# Bug report
This was not reported as an issue in the wild, but as a hypothetical when discussion the impacts of drop in files [on Launchpad](https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/2088207).

Cloud-init uses a hard-coded drop-in file at `/etc/ssh/sshd_config.d/50-cloud-init.conf` to update the sshd configuration. Pre-existing drop-in files may supersede this file. Cloud-init's implementation is buggy in the case that another drop-in file takes priority.