Find overrides of variables with names in their dots

[jsoriano]

Variables in policy templates can have dots in their names. This names are defined as plain strings in yaml files. For example like this:

      - name: jmx.mappings
        type: yaml
        title: JMX Mappings
        multi: false
        required: true
        show_user: true

These variables can be overridden in system tests configuration files, for example like this:

vars:
  hosts:
    - {{Hostname}}:{{Port}}
  jmx.mappings: |
    - mbean: 'java.lang:type=Runtime'
      attributes:
        - attr: Uptime
          field: uptime

These configuration files are parsed with go-ucfg, configured to use . as path separator, so variable names containing dots are expanded. Also, when parsed, they are parsed into an opaque VarVariable structure, so it is not possible to access their content, so overrides cannot be defined for variables with dots in their names.
This happens for example in elastic/integrations#4706.

This issue could be easily solved if ucfg would support preserve as defined in elastic/go-ucfg#124.

This change introduces a hacky workaround to give access to child elements, so they can be also used for overrides.

Alternatives considered:

• Completely disable variable expansion when parsing this configuration. This could produce breaking changes, and could be misleading, as developers are used to this kind of configurations where handling of objects structure is transparent for them.
• Implement preserve as defined in Control Structure on unpack go-ucfg#124. Probably a rabbit hole.
• Parse the configuration as common.MapStr instead of map[string]VarValue, and generate the VarValues later. This could be cleaner, but require some refactors. I may give it a try though.

[elasticmachine]

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• Start Time: 2022-11-23T16:22:52.139+0000

• Duration: 6 min 37 sec

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Passed	458
Skipped	0
Total	458

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[jsoriano]

• Parse the configuration as common.MapStr instead of map[string]VarValue, and generate the VarValues later. This could be cleaner, but require some refactors. I may give it a try though.

Ok, this ended up being much more clear: #1049.

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• Start Time: 2022-11-23T16:26:36.975+0000

• Duration: 28 min 2 sec

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Test	Results
Failed	0
Passed	866
Skipped	0
Total	866

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