Pass runners.config through the template engine

What does this MR do?

Passes runners.config through the template engine

Why was this MR needed?

To use helm variable in config.template.toml

What's the best way to test this MR?

1. Set runners config with helm variable

runners:
  config: |
    [[runners]]
      [runners.kubernetes]
        image = "ubuntu:16.04"
      [runners.kubernetes.pod_labels]
        release = "{{ .Release.Name }}"

2. Run helm template to see result

helm template myReleaseName <path to chart>

  config.template.toml:   |
    [[runners]]
      [runners.kubernetes]
        image = "ubuntu:16.04"
      [runners.kubernetes.pod_labels]
        release = "myReleaseName"

What are the relevant issue numbers?

Issue 237