Support Jinja2's ChainableUndefined to reduce attribute errors

[hackery]

A common need in Jinja rendered states and files is to access nested attributes of pillar data, but you have to take steps to avoid referencing any undefined values in the chain with many non-trivial merged pillars - using myapp.cfg.pipeline.credentials.msk.keystore needs multiple levels of guard to avoid 'dict object' has no attribute 'msk' type errors. The default(...) construct doesn't help except when used for top-level items.

Describe the solution you'd like
Any nested element in chained attribute access which evaluates as Undefined should not throw an error, but continue to evaluate the whole reference as Undefined.

Jinja has supported a ChainableUndefined environment option since v2.11 in early 2020 which provides exactly this functionality at minimal implementation expense for the Salt project. The one significant complexity I foresee is how this should be optionally enabled - whether it can only be a global option, or should have a way to localise it to target states/files (and whether this applies transitively to include/imported files, rendered managed files etc).

Describe alternatives you've considered

• Multiple levels of guard on all references - makes files much less readable/maintainable
• Exhaustive setting of default values everywhere in pillar data - not always appropriate, bloats data and reduces maintainability

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