kea: T3316: T6163: Ensure correct permissions on lease files

[sarthurdev]

Change Summary

Ensure correct permissions on lease files before service start

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)

• https://vyos.dev/T6163

Related PR(s)

Component(s) name

dhcp-server, dhcpv6-server

Proposed changes

Iterates the lease files present and ensure ownership by _kea user.

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