Support running helm dependency update

[MattHodge]

Adds a new parameter update_dependencies to the helm_resource extension. When set to try, a helm dependency update will be run before the helm install on the given chart.

This is useful if you have a chart with file based dependencies eg:

apiVersion: v2
name: foo
type: application
version: 0.0.1

dependencies:
  - name: "bar"
    repository: file://../../templates/bar
    version: 0.0.1

Without this, when you make changes to the dependent chart bar, they will not be shown in the rendered helm chart.

I tried just passing --dependency-update flag to the helm install command, but this doesn't work.