configd: T3411: Extend redirect_stdout to subprocesses

[jestabro]

Change Summary

Extend the redirect_stdout contextmanager in vyos-configd to redirect stdout from subprocesses.

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)

• https://phabricator.vyos.net/T3411

Component(s) name

Proposed changes

Instead of using the contextlib provided redirect_stdout, we provide a simple contextmanager which will additionally redirect stdout from subprocesses.

How to test

touch /config/vyos.ifconfig.debug
set interfaces ethernet eth0 description 'TEST'
[one will see full debug info in console]
reboot
[one will find full debug info in /tmp/vyos-configd-script-stdout]

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