Ansible command task failure when listing iptables rules in persistent-firewall role

Occasionally we see the persistent-firewall role fail to list iptables rules with iptables-save using an ansible command task.

This is what it looks like in the zuul-executor log:

2019-09-30 14:37:25,375 DEBUG zuul.AnsibleJob.output: [e: 53c73870b5b343b6a6c883a388b0070d] [build: bafbb86f7b924f35a595edd2d4aa7196] Ansible output: b'ok: [primary] => {"changed": false, "failed_when_result": false, "module_stderr": "", "module_stdout": "", "msg": "MODULE FAILURE\\nSee stdout/stderr for the exact error", "rc": -13}'

The one "upside" to this problem is that it happens in the pre-run playbook so jobs are retried when it happens and it doesn't happen on every run. As a note it appears to happen on multiple distros, on multiple clouds, and in multiple jobs so nothing specific to point at there.

Adding this bug so that we can track the issue with e-r.