fix(datasource): no SSH keys for an address without a reverse DNS entry

- A booting VM is named by its address's reverse DNS entry. Without one,
  the address itself was taken as the name, so its first number ("10",
  "192") named the VM: a user who made a VM with that name had their keys
  given to every such VM. Such an address now gets no keys; its metadata
  hostname is still the address.
- Keys are written as quoted YAML strings, so a comment with an apostrophe
  or ": " no longer breaks the key or the file.

datasource/templates/user_data

@@ -1,6 +1,7 @@
 #cloud-config
-{% if instance_keys %}
+{% autoescape off %}{% if instance_keys %}
 ssh_authorized_keys:
 {% for key in instance_keys %}  - {{ key }}
 {% endfor %}
 {% endif %}
+{% endautoescape %}