dhcp: T6611: add vendor support for ShoreTel IP phones

[bk2zsto]

Change Summary

Add ShoreTel support to dhcp

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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

dhcp

Proposed changes

update KEA initial conf and interface def

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