Some pre-upgrade validations failed during FFU to 17.1 due to Permission denied

Issue

• Some pre-upgrade validations failed due to /var/lib/mistral/.ssh/tripleo-admin-rsa public key is not accessible.
• All validations that the Undercloud is included in the target host list failed.
  • container-status
  • undercloud-sysctl
  • openstack-endpoints
  • undercloud-service-status
  • undercloud-ipa-server-check
  • validate-selinux
  • system-encoding
  • stack-health
  • undercloud-disabled-services
  • check-rsyslog
  • service-status
  • image-serve
  • package-version
  • ntp
  • repos
  • check-reboot
  • collect-flavors-and-verify-profiles
  • tripleo-latest-packages-version
  • undercloud-disk-space-pre-upgrade
• The below is the logs when the undercloud-sysctl validation was executed manually.

(undercloud) [stack@undercloud-0 ~]$ ansible-playbook -i inventory.yaml /usr/share/ansible/validation-playbooks/undercloud-sysctl.yaml 

PLAY [undercloud] *********************************************************************************************************************************************

TASK [Gathering Facts] ****************************************************************************************************************************************
fatal: [undercloud]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: load pubkey \"/var/lib/mistral/.ssh/tripleo-admin-rsa\": Permission denied\r\nload pubkey \"/var/lib/mistral/.ssh/tripleo-admin-rsa\": Permission denied\r\nWarning: Permanently added 'localhost' (ECDSA) to the list of known hosts.\r\nno such identity: /var/lib/mistral/.ssh/tripleo-admin-rsa: Permission denied\r\ntripleo-admin@localhost: Permission denied (publickey,password).", "unreachable": true}

PLAY RECAP ****************************************************************************************************************************************************
undercloud                 : ok=0    changed=0    unreachable=1    failed=0    skipped=0    rescued=0    ignored=0