feat(parser): support ref-tags with default values

[MatteoVoges]

Proposed Changes

• Added support for specifying default values for non existing keys / dictionary-paths
• Extended functionality to support a reference-tag as default value, e.g. ${spec:value||${default:value||another-default}}
• Use || as delimiter, because it fits well in the kapitan ecosystem

Examples

parameters:
  existing:
    key: myvalue

  foo: ${non:existing:key||bar} # --> bar (default value)
  mykey: ${existing:key||bar} # --> myvalue (value exists)

parameters:
  defaults:  
    default1: myDefault

  spec:
    spec1: ${not:existing||${defaults:default1}} # --> myDefault (tag in default-value exists)
    spec2: ${not:existing||${defaults:notfound||fallback}} # --> fallback (tag in default-value not exists, default gets taken) 

Breaking changes

• You should not have the || - delimiter in your yaml-keys anymore!

To Do

• Add tests
• Add documentation (also in kapitan docs)
  @ademariag Where should this be done?

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Contributed by

[MatteoVoges]

This is a replacement for #11 , because of bad references on the branch

[ademariag]

@MatteoVoges for now add it here: https://github.com/kapicorp/reclass/blob/develop/README-extensions.rst

[gburiola]

• Very good work. Please also add the 4 cases from your examples above to the tests

[MatteoVoges]

• Very good work. Please also add the 4 cases from your examples above to the tests

Thank you, it was quite hard to find the correct spot where the tests need to go, because the nested interpolation logic doesn't happen in the modules where the feature comes to action... But I think its fine to do it in the parameters tests