T5319: remove workarounds for incorrect defaults in config-mode scripts

[jestabro]

Change Summary

This updates all conf_mode scripts to remove the workarounds for incorrect defaults, and replace any other calls of defaults with reliable patterns of merging default values in the config_dict.

Note this was tested without commit 710dac5, pending a fix for the revised smoketests, although it has been rebased over current as is.

In the course of converting conf_mode scripts, another pattern for merging defaults was introduced in T5443, as intermediary between the coarse-grained control of automatic merging of defaults in get_config_dict and the fine-grained control of get_config_defaults/config_dict_merge; the available patterns are summarized in that task, and examples of each are in the conf_mode script translations. That method is included in this PR.

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/T5319
• https://vyos.dev/T5443
• https://vyos.dev/T5434
• https://vyos.dev/T5435

Component(s) name

Proposed changes

How to test

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