executor_1      |   cmdline: git clone ssh://git@github.ibm.com/wrigley/zuul-experiment-config.git /var/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config
executor_1      |   stderr: 'Cloning into '/var/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config'...
executor_1      | Host key verification failed.

But when I use the key file from /etc/zuul/zuul.conf:
[gearman]
server=scheduler

[gearman_server]
start=true

[zookeeper]
hosts=zk

[scheduler]
tenant_config=/etc/zuul/main.yaml

[connection "gerrit"]
name=gerrit
driver=gerrit
server=gerrit
sshkey=/var/ssh/zuul
user=zuul
password=secret
baseurl=http://gerrit:8080
auth_type=basic

[connection "github.ibm.com"]
driver = github
server = github.ibm.com
api_token = 980ca2437b0de08a363657eea3eeda9114ed7dc8
webhook_token = pHQizqZRxnGQk
sshkey = /opt/zuul_keys/github.ibm.com/id_rsa

[connection "zuul-ci.org"]
name=zuul-ci
driver=git
baseurl=https://git.zuul-ci.org/

[connection "mysql"]
name=mysql
driver=sql
dburi=mysql+pymysql://zuul:secret@mysql/zuul

[web]
listen_address=0.0.0.0

[executor]
private_key_file=/var/ssh/nodepool
default_username=root
trusted_rw_paths=/srv/static/logs

like as follows:
/usr/src # ssh-agent sh -c ‘ssh-add /opt/zuul_keys/github.ibm.com/id_rsa; git clone ssh://git@github.ibm.com/wrigley/zuul-experiment-config.git /va
r/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config’
Identity added: /opt/zuul_keys/github.ibm.com/id_rsa (/opt/zuul_keys/github.ibm.com/id_rsa)
Cloning into ‘/var/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config’...
remote: Counting objects: 14, done.
remote: Total 14 (delta 0), reused 0 (delta 0), pack-reused 14
Receiving objects: 100% (14/14), done.


It works. So what keyfile it using?