Support the ability to fork manifests into specific locations of a chart

[aiyengar2]

Add the following field to Charts and AdditionalCharts to support forking manifests

manifests:
- url: <required> # UpstreamOptions
  subdirectory: <optional> # UpstreamOptions
  commit: <optional> # UpstreamOptions
  filepath: <optional> # a specific file in the subdirectory to pull in the manifest for
  manifestPath: <required> # Where to drop the manifests, directories along path will be created if they don't exist

[aiyengar2]

This feature could be used by Monitoring V2 to maintain the latest manifest for nginx-ingress + traefik as part of https://github.com/rancher/tasks/issues/69.

[aiyengar2]

While we cannot fork manifests into a specific location within a chart yet, we will add support for setting manifests as an upstream option.

[aiyengar2]

With the new experimental feature in v0.3.x that allows users to point to a manifest to treat as a dummy Helm chart, this would be a great idea to prioritize sometime soon.