Use blockinfile for tripleo-ssh-known-hosts

lineinfile combined with with_items was very inefficient for this task.
Given that each node's hostkey is added to every other host, it resulted
in O(n²) performance.

Additional networks per node also worsened the problem. For example, in
a 100 node deployment, with 4 networks per node, this task would need to
be executed 40,000 times (100 * 100 * 4).

Switching to use blockinfile brings the performance back to O(n), and
also removes any dependency on the number of networks per node.

Change-Id: Id51d27f53abf3421c29a11065377e9303ad68d79

tripleo_ansible/roles/tripleo-ssh-known-hosts/tasks/main.yml

@@ -61,11 +61,10 @@
           {% endfor %}
 
     - name: Add host keys to temporary ssh_known_hosts
-      lineinfile:
+      blockinfile:
         path: "{{ ssh_known_hosts_tmp.path }}"
-        line: "{{ item }}"
+        block: "{{ ssh_known_hosts_lines }}"
         create: true
-      with_items: "{{ ssh_known_hosts_lines.splitlines() }}"
 
     # Workaround https://bugs.launchpad.net/tripleo/+bug/1810932
     # Ansible modules perform a replace instead of in-place modification.