T5453: nat66: fix nat66 which got broken after nat load-balance was introduced

[nicolas-fort]

Change Summary

Exclude checks for nat load-balance when using ipv6 while parsing nat rules.

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/T5453

Component(s) name

nat nat66

Proposed changes

Exclude checks for nat load-balance when using ipv6 while parsing nat rules.

How to test

root@HASH:/usr/libexec/vyos/tests/smoke/cli# ./test_nat66.py 
test_destination_nat66 (__main__.TestNAT66.test_destination_nat66) ... ok
test_destination_nat66_prefix (__main__.TestNAT66.test_destination_nat66_prefix) ... ok
test_destination_nat66_protocol (__main__.TestNAT66.test_destination_nat66_protocol) ... ok
test_destination_nat66_without_translation_address (__main__.TestNAT66.test_destination_nat66_without_translation_address) ... ok
test_nat66_no_rules (__main__.TestNAT66.test_nat66_no_rules) ... ok
test_source_nat66 (__main__.TestNAT66.test_source_nat66) ... ok
test_source_nat66_address (__main__.TestNAT66.test_source_nat66_address) ... ok
test_source_nat66_protocol (__main__.TestNAT66.test_source_nat66_protocol) ... ok
test_source_nat66_required_translation_prefix (__main__.TestNAT66.test_source_nat66_required_translation_prefix) ... 
Source NAT66 configuration error in rule 5: outbound-interface not
specified


Source NAT66 configuration error in rule 5: translation address not
specified

ok

----------------------------------------------------------------------
Ran 9 tests in 14.567s

OK
root@HASH:/usr/libexec/vyos/tests/smoke/cli# 

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