haproxy: T5222: Enable backend completion in service ruleset

[indrajitr]

Change summary

In CLI, enable completion for backend in haproxy service ruleset like so:

set load-balancing haproxy service NAME rule 10 set backend

Currently, it doesn't support completion for existing haproxy backends.

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/T5222

Related PR(s)

How to test / Smoketest result

Before:

vyos@test-0106# set load-balancing haproxy service MyService rule 10 set backend 
Possible completions:
   <text>               Backend name
     

After:

vyos@test-0106# set load-balancing haproxy service MyService rule 10 set backend
Possible completions:
   <text>               Backend name
   my-backend     

      
[edit]

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

[github-actions]

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[github-actions]

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Details

CI logs

• CLI Smoketests (no interfaces) ❌ failed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests ❌ failed
• RAID1 tests 👍 passed
• TPM tests 👍 passed