use the next available port for wireguard

[mattkasun]

Describe your changes

check if WgPort is available, if not find a free port

Issue ticket number and link

fixes issue #546

Checklist

• [ ] Is it a bug fix
• [ ] Is a typo/documentation fix
• [x] Is a feature enhancement
• [ ] It is a refactor
• [ ] Created tests that fail without the change (if possible)
• [ ] Extended the README / documentation, if necessary

[CLAassistant]

All committers have signed the CLA.