[env] Update VPN environment variables to OPENVPN for WireGuard suppo…

[Adityashandilya555]

…rt #227

Changed VPN_* environment variables to OPENVPN_* to distinguish them from upcoming WireGuard VPN configurations. This prepares the codebase for WireGuard support by making OpenVPN-specific variables clearly identifiable.

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #227

Description of Changes

• VPN_DOMAIN → OPENVPN_DOMAIN
• VPN_NAME → OPENVPN_NAME
• VPN_CLIENT_NAME → OPENVPN_CLIENT_NAME

Updated across configuration files, Docker images, scripts, and documentation.

Screenshot

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