openvpn: T6374: ensure that TLS role is configured for site-to-site with TLS (backport #3528)

[mergify]

Change Summary

OpenVPN site-to-site mode (still) can work without TLS, so it can work without tls role — but we need to make sure that if it's using TLS rather than PSKs, TLS role is configured.

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

OpenVPN.

Proposed changes

How to test

Smoketest result

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

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This is an automatic backport of pull request #3528 done by [Mergify](https://mergify.com).