nat64: T6627: call check_kmod within standard config function (backport #3927)

[mergify]

Change Summary

The config mode script nat64.py calls the modprobe utility check_kmod outside of the standard functions, leading to an OSError when run under configd. Current behavior of configd will re-run the script within the CLI context, leading to config and smoketest success; as that behavior will be changed in the future to accommodate all scripts running within the configd context (T6608), correct now.

There are several other config scripts that use this pattern, which will be all changed in a separate PR. The nat64.py script deserved special attention as it provides an example of state configuration informing the syntax verification stage, which should be kept in mind as we move to distinguishing syntax verification from system state verification.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Related PR(s)

Component(s) name

Proposed changes

How to test

Confirmation provided in task. Success in smoketest and journalctl output, below.

Smoketest result

vyos@vyos:/usr/libexec/vyos/tests/smoke/cli$ ./test_nat64.py
test_snat64 (__main__.TestNAT64.test_snat64) ... ok

----------------------------------------------------------------------
Ran 1 test in 3.788s

OK
vyos@vyos:/usr/libexec/vyos/tests/smoke/cli$ journalctl -u vyos-configd --no-pager

...
Aug 01 16:43:35 vyos vyos-configd[4299]: Received message: {"type": "node", "last": true, "data": "/usr/libexec/vyos/conf_mode/nat64.py"}
...
Aug 01 16:43:35 vyos vyos-configd[4299]: Sending response 1
...

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

---

This is an automatic backport of pull request #3927 done by [Mergify](https://mergify.com).

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