make known_hosts idempotent for OOD login host

Author: sjpb

ansible/roles/openondemand/tasks/main.yml

@@ -133,11 +133,14 @@
   command:
     cmd: "ssh-keyscan -t ssh-ed25519 {{ openondemand_clusters.slurm.v2.login.host }}"
   register: _openondemand_login_key
+  changed_when: false
 
 - name: Add login hostkey to known hosts
-  copy:
-    dest: /etc/ssh/known_hosts
-    content: "{{ _openondemand_login_key.stdout }}"
+  lineinfile:
+    path: /etc/ssh/known_hosts
+    create: true
+    line: "{{ _openondemand_login_key.stdout }}"
+    regexp: "^{{ openondemand_clusters.slurm.v2.login.host}}"
     owner: root
     group: root
     mode: o=rw,go=r