This way, we don't have to rely on user configuration to include
known_hosts entries for the deployments.

It makes `nixops import --include-keys` unnecessary, unless you
use those entries outside of nixops.

Since recently we can get our deployment state from remote storage
backends, but we didn't have a way to get configure the known_hosts
yet. This is now largely unnecessary.

This functionality requires some cooperation from the plugins. For
instance, here's what ec2 needs to do: (pun intended)

+    def get_ssh_host_keys(self):
+        return self.private_ipv4 + " " + self.public_host_key + "\n" + self.public_ipv4 + " " + self.public_host_key + "\n"

…present

Thanks Mic92!