T7973: do not disallow all-zero host part IPv6 host addresses

[dmbaturin]

Change Summary

As discussed in the task, all-zero host part IPv6 addresses are valid unicast addresses and other network vendors including Cisco and Arista allow them, and so should we.

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)
• Other (please describe):

Related Task(s)

Related PR(s)

Proposed changes

How to test

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[jestabro]

Drop restriction: logic is clear; simple test passes; integration tests included.

[sever-sever]

Manually tested

vyos@r14# run show conf com | match dum
set interfaces dummy dum1 address '2001:db8::1/64'
set interfaces dummy dum5 address '2001:db8::/32'
set interfaces dummy dum10 address '2001:db8::/128'
[edit]
vyos@r14# 
[edit]
vyos@r14# run show interfaces 
Codes: S - State, L - Link, u - Up, D - Down, A - Admin Down
Interface    IP Address         MAC                VRF        MTU  S/L    Description
-----------  -----------------  -----------------  -------  -----  -----  -------------
dum1         2001:db8::1/64     b6:f1:ac:db:59:2a  default   1500  u/u
dum5         2001:db8::/32      82:ad:57:e0:0a:b8  default   1500  u/u
dum10        2001:db8::/128     62:b3:8e:5e:28:a1  default   1500  u/u