Stricter sshd config

user_data

 #cloud-config
 
 write_files:
+  - path: /etc/ssh/sshd_config
+    permissions: 0600
+    owner: root:root
+    content: |
+      # Use most defaults for sshd configuration.
+      UsePrivilegeSeparation sandbox
+      Subsystem sftp internal-sftp
+      PermitRootLogin no
+      AllowUsers core
+      PasswordAuthentication no
+      ChallengeResponseAuthentication no
   - path: /etc/sysctl.d/libresh.conf
     permissions: 0644
     owner: root