vyos.template: T3664: use a module-level global variable for the default template directory (backport #3415)

[mergify]

Change Summary

Use a module-level global variable for the default template directory as a more convenient and secure alternative to environment variable. It appears to propagate correctly. This removes the concern about malicious users injecting stuff into scripts on a live system by changing the environment variables.

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): internal change.

Related Task(s)

Related PR(s)

Component(s) name

vyos.template

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 #3415 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 9aa8c3c has failed:

On branch mergify/bp/sagitta/pr-3415
Your branch is up to date with 'origin/sagitta'.

You are currently cherry-picking commit 9aa8c3cd9.
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Unmerged paths:
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	both modified:   python/vyos/template.py

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