ntp: T4456: support listening on specified interface (equuleus)

[c-po]

Change Summary

When clients only use DHCP for interface addressing we can not bind NTPd to
an address - as it will fail if the address changes. This commit adds support
to bind ntpd to a given interface in addition to a given address.

set system ntp interface <name>

(cherry picked from commit 6732df1)

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://phabricator.vyos.net/T4456

Component(s) name

Proposed changes

How to test

Integrated Smoketests

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