Add the same logic as for virt hosts to cloud spin up. When new instance is made it gathers the key automatically.

tasks/persistent_cloud_new.yml

@@ -44,6 +44,20 @@
   local_action: wait_for host={{ public_ip }} port=22 delay=1 timeout=600
   when: host_is_up|failed
 
+- name: gather ssh host key from new instance
+  local_action: command ssh-keyscan -t rsa {{ inventory_hostname }}
+  ignore_errors: True
+  register: hostkey
+  when: host_is_up|failed
+
+- name: add new ssh host key (you still need to add it to official ssh_host_keys later)
+  local_action: known_hosts path={{item}} key="{{ hostkey.stdout }}" host={{ inventory_hostname }} state=present
+  ignore_errors: True
+  with_items:
+  - /root/.ssh/known_hosts
+  - /etc/ssh/ssh_known_hosts
+  when: host_is_up|failed
+
 # SSH is up and running, however cloud-init still did not deployed ssh keypair
 # we have to wait some time. 10 sec is usually enough, but not always.
 - name: waiting for cloud-init

tasks/transient_cloud.yml

@@ -36,6 +36,18 @@
 - name: wait for he host to be hot
   local_action: wait_for host={{ nova_result.public_ip }} port=22 delay=1 timeout=600
 
+- name: gather ssh host key from new instance
+  local_action: command ssh-keyscan -t rsa {{ nova_result.public_ip }}
+  ignore_errors: True
+  register: hostkey
+
+- name: add new ssh host key (you still need to add it to official ssh_host_keys later)
+  local_action: known_hosts path={{item}} key="{{ hostkey.stdout }}" host={{ nova_result.public_ip }} state=present
+  ignore_errors: True
+  with_items:
+  - /root/.ssh/known_hosts
+  - /etc/ssh/ssh_known_hosts
+
 # SSH is up and running, however cloud-init still did not deployed ssh keypair
 # we have to wait some time. 10 sec is usually enough, but not always.
 - name: waiting for cloud-init