Support force fetch per compile target

[Moep90]

Support the ability to force fetch selected external dependencies.

The documentation says: "If you simply want the latest chart available, either don't include the version key or specify an empty string."

But to have the latest to compile, I need to always run kapitan compile --fetch --force.
This applies to all external dependencies.

Helm

This would fetch the latest version of the chart if there is an update.
Make sure to run helm helm repo update prior to fetching the latest chart.

  kapitan:
    dependencies:
    - type: helm
[...]
      version: "" <----- to fetch always latest
      fetch_always: true <--- force download this dependency always

Git

This would apply if we update the master between kapitan runs.

- type: git output_path: source/kapitan
  source: git@github.com:kapicorp/kapitan.git
  subdir: kapitan
  ref: master
  fetch_always: true <--- force download this dependency always

HTTP

- type: http | https
  output_path: path/to/file
  source: http[s]://<url>
  unpack: True | False
  fetch_always: true <--- force download this dependency always

Full example with helm

---
parameters:
  filebeat:
    chart_name: filebeat
    fullname: filebeat
    chart_version: "7.17.3"
    application_version: ${filebeat:chart_version}
    namespace: ${namespace}
  kapitan:
    dependencies:
    - type: helm
      output_path: components/charts/${filebeat:chart_name}/${filebeat:chart_version}/${filebeat:application_version}
      source: https://helm.elastic.co
      version: "" <----- to fetch always latest
      fetch_always: true <--- force download this dependency always
      chart_name: ${filebeat:chart_name}
    compile:
    - input_type: helm
      input_paths:
        - components/charts/${filebeat:chart_name}/${filebeat:chart_version}/${filebeat:application_version}
      output_path: manifests
      helm_params:
        namespace: ${filebeat:namespace}
        name: ${filebeat:chart_name}
        output_file: ${filebeat:chart_name}.yml
      helm_values: ${filebeat:helm_values}

Example compile run

$ kapitan compile --fetch
Not updating lib/terraform/__init__.py (file already exists)
[...]
Dependency helm chart filebeat and version 7.17.3: fetching now
Dependency helm chart filebeat and version 7.17.3: successfully fetched

[Moep90]

closed by #862