Replace custom serial loop with throttle

We can use throttle now that we require Ansible 2.9.

Change-Id: I567e83cac4d5e80bd097b30ddefa0cc188118ad2

ansible/roles/ssh-known-host/tasks/main.yml

@@ -29,7 +29,7 @@
     module: known_hosts
     host: "{{ item[0].item }}"
     key: "{{ item[1] }}"
-  # TODO(priteau): Replace the following with "throttle: 1" once we require
-  # Ansible >= 2.9
-  loop: "{{ query('subelements', ansible_play_batch | map('extract', hostvars, ['keyscan_result', 'results']) | map('first') | list, 'stdout_lines') }}"
-  run_once: True
+  with_subelements:
+    - "{{ keyscan_result.results }}"
+    - stdout_lines
+  throttle: 1