Description of problem:
The following error is raised during Overcloud FFU prepare:
2021-01-31 22:15:54 | fatal: [undercloud]: UNREACHABLE! => {"changed": false, "msg": "Data could not be sent to remote host \"localhost\". Make sure this host can be reached over ssh: Warning: Permanently added 'localhost' (ECDSA) to the list of known hosts.\r\nno such identity: /home/stack/.ssh/id_rsa: No such file or directory\r\ntripleo-admin@localhost: Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).\r\n", "unreachable": true}

It had been reported before in this Jira ticket: https://projects.engineering.redhat.com/browse/UPG-2562


It was found in several FFU jobs, all of them upgrading to the latest 16.1 build, RHOS-16.1-RHEL-8-20210129.n.0:
http://rhos-ci-logs.lab.eng.tlv2.redhat.com/logs/rcj/DFG-upgrades-ffu-ffu-upgrade-13-16.1_director-rhel-virthost-3cont_3db_3msg_2net_2comp_3ceph-ipv6-vxlan-composable/81/undercloud-0/home/stack/overcloud_upgrade_prepare_containers.log.gz
http://rhos-ci-logs.lab.eng.tlv2.redhat.com/logs/rcj/DFG-upgrades-ffu-ffu-upgrade-13-16.1_director-rhel-virthost-3cont_2comp-ipv6-vxlan-HA-no-ceph/90/undercloud-0/home/stack/overcloud_upgrade_prepare_containers.log.gz
http://rhos-ci-logs.lab.eng.tlv2.redhat.com/logs/rcj/DFG-upgrades-ffu-ffu-upgrade-13-16.1_director-rhel-virthost-3cont_2comp_1ipa-ipv4-vxlan-HA-no-ceph-tls-everywhere/52/undercloud-0/home/stack/overcloud_upgrade_prepare_containers.log.gz
http://rhos-ci-logs.lab.eng.tlv2.redhat.com/logs/staging/DFG-upgrades-ffu-ffu-upgrade-13-16.1_director-rhel-virthost-3cont_2comp_2net-ipv4-geneve-HA-no-ceph-ovn-vlan-provider-network/18/undercloud-0/home/stack/overcloud_upgrade_prepare_containers.log.gz


I compared the build failing with the previous build for the same job and I think I found something that could explain the issue:
fails (http://rhos-ci-logs.lab.eng.tlv2.redhat.com/logs/rcj/DFG-upgrades-ffu-ffu-upgrade-13-16.1_director-rhel-virthost-3cont_3db_3msg_2net_2comp_3ceph-ipv6-vxlan-composable/81/undercloud-0/var/lib/mistral/a3c756f0-4ca5-4805-9c6a-dad1014140e9/inventory.yaml.gz):
Undercloud:
  hosts:
    undercloud: {}
  vars:
    ansible_connection: ssh
    ansible_host: localhost
    ansible_python_interpreter: /usr/bin/python3
    ansible_remote_tmp: /tmp/ansible-${USER}
    ansible_ssh_private_key_file: /home/stack/.ssh/id_rsa
    ansible_ssh_user: tripleo-admin

passes (http://rhos-ci-logs.lab.eng.tlv2.redhat.com/logs/rcj/DFG-upgrades-ffu-ffu-upgrade-13-16.1_director-rhel-virthost-3cont_3db_3msg_2net_2comp_3ceph-ipv6-vxlan-composable/80/undercloud-0/var/lib/mistral/e8cbadc2-a691-4d5f-afa2-1f7237431630/inventory.yaml.gz):
Undercloud:
  hosts:
    undercloud: {}
  vars:
    ansible_connection: ssh
    ansible_host: localhost
    ansible_python_interpreter: /usr/bin/python3
    ansible_remote_tmp: /tmp/ansible-${USER}
    ansible_ssh_private_key_file: /var/lib/mistral/.ssh/tripleo-admin-rsa
    ansible_ssh_user: tripleo-admin


I checked this on my env after the FFU job failed:
[root@undercloud-0 ~]# ssh -i /home/stack/.ssh/id_rsa tripleo-admin@localhost
tripleo-admin@localhost's password:

[root@undercloud-0 ~]# ssh -i  /var/lib/mistral/.ssh/tripleo-admin-rsa tripleo-admin@localhost
[tripleo-admin@undercloud-0 ~]$





Version-Release number of selected component (if applicable):
RHOS-16.1-RHEL-8-20210129.n.0
It didn't fail with previous 16.1 build RHOS-16.1-RHEL-8-20210120.n.1

How reproducible:
it seems it happens 100%

Steps to Reproduce:
1. run any? 13-16.1 ffu job
2.
3.