Fix override of variables with dots in their names in system tests

[jsoriano]

Use common.MapStr to be able to get variables independently of them being expanded because of having dots or not.

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.

[elasticmachine]

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• Start Time: 2022-11-23T16:34:52.271+0000

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[mrodm]

LGTM