Add GCEState.get_ssh_host_keys

This will let NixOps provide the host keys directly to the ssh
client, regardless of whether they've been saved to user dotfiles.

It solves a problem where the host keys were not known on systems
that retrieve the state from a remote state provider.

See NixOS/nixops#1464

nixops_gcp/backends/gce.py

@@ -207,6 +207,19 @@ def gen_metadata(self, metadata):
  "items": [{"key": k, "value": v} for k, v in metadata.items()],
  }
 
+ # invoked by nixops before calling ssh
+ def get_ssh_host_keys(self):
+ return (
+ self.private_ipv4
+ + " "
+ + self.public_host_key
+ + "\n"
+ + self.public_ipv4
+ + " "
+ + self.public_host_key
+ + "\n"
+ )
+
  def update_block_device_mapping(self, k, v):
  x = self.block_device_mapping
  if v == None: