fix(template): support template strings in ConfigTemplate.configs

[vvagaytsev]

What this PR does / why we need it:

Prior to this PR Garden did not allow the templating in the ConfigTemplate.configs like this:

kind: ConfigTemplate
name: runs-template
inputsSchemaPath: schema.json

configs:
  - $concat:
      $forEach: ${inputs.names}
      $return:
        kind: Run
        type: exec
        name: "${item.value}"
        spec:
          command: ["echo", "${item.value}"]

---
kind: RenderTemplate
template: runs-template
name: my-runs
inputs:
  names: ["run-a", "run-b"]

and it failed with error:

Unexpected kind 'undefined' found in ConfigTemplate 'runs-template'. Supported kinds are: Build, Deploy, Run, Test and Workflow

Click to expand schema.json

{
  "type": "object",
  "properties": {
    "names": {
      "type": "array"
    }
  }
}

Now, Garden partially resolves the configs field of a given ConfigTemplate.