[Proposal] Add useHelmSecrets option to helm config

[RoryShively]

I'm rewriting this because it hasn't received enough attention. I originally had proposed to add plugins to the helm config but my proposed implementation was quite complicated. This new proposal has been greatly simplified

I created a PR for helm-secrets that will make integration with skaffold easier.
zendesk/helm-secrets#67

We can create a config option that will use helm-secrets if set to true to decrypt secrets on the fly before deployment. The config might look like this:

apiVersion: skaffold/v1alpha4
kind: Config
build:
  ...
deploy:
  helm:
    releases:
    - name: helm-secrets-test
      chartPath: helm_secrets_test
      valuesFiles:
      - helm_secrets_test/helm_vars/values.yaml
      - helm_secrets_test/helm_vars/secrets.yaml
      useHelmSecrets: true

All that needs to change in the skaffold code is pkg/skaffold/deploy/helm line 110

	args := append([]string{"secrets", "--kube-context", h.kubeContext}, arg...)

Add secrets as an argument if useHelmSecrets is true

[nkubala]

I'm a +1 on this. @RoryShively would you like to send a PR?

[RoryShively]

@nkubala Yeah. Sorry about the late reply, I'll get you something by the end of the day hopefully

[RoryShively]

@nkubala Just submitted a first draft
#1617

• Placed useHelmSecrets option in latest schema
• prepended "secrets" to args for helm deployment if useHelmSecrets selected

[nkubala]

this should be fixed through #1617, thanks @RoryShively!