save-config: T4292: rewrite vyatta-save-config.pl to Python

[jestabro]

Change Summary

This is a draft PR, pending the merge of https://github.com/vyos/vyatta-cfg/pull/50 without which the original problem of allowing whitespace in tag node value names will be exacerbated by using config_tree for save-config. Also, there will need to be the supporting PR's for vyatta-cfg and vyatta-config-mgmt to use the rewrite vyos-save-config; they are listed below, and will need to be merged at the same time.

https://github.com/vyos/vyatta-cfg/pull/51 https://github.com/vyos/vyatta-config-mgmt/pull/12

This PR replaces https://github.com/vyos/vyos-1x/pull/1328, which will be closed. The reasons for dropping the updated load-config in the sequence of commits are summarized in: https://phabricator.vyos.net/T4295 Namely, using the external functions my_set/my_delete in place of the internal cstore setCfgPath/deleteCfgPath incured a notable performance hit, that will be addressed by another method; note that any approach does not pertain to loading the config at boot.

Consequently, this PR includes only the refactoring of component string read/write, and the rewrite of vyatta-save-config.pl.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Code style update (formatting, renaming)
• [X] Refactoring (no functional changes)
• [X] 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/Txxxx

Component(s) name

Proposed changes

How to test

Checklist:

• [X] I have read the CONTRIBUTING document
• [X] I have linked this PR to one or more Phabricator Task(s)
• [X] I have run the components SMOKETESTS if applicable
• [X] My commit headlines contain a valid Task id
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly