Add support for providing env vars with envFrom

[ghost]

Motivation

We include the localstack helm chart as a dependency to a wrapper chart and would like to be able to set an environment variable that is derived from a value passed to the wrapper. This is currently not possible as the values for the localstack subchart can not refer to another value. Here's a brief example of how we'd like to use this:

Chart.yaml:

apiVersion: v2
name: wrapper-chart
description: A wrapper around the localstack chart
type: application
version: 1.0.0
appVersion: "1.0.0"
dependencies:
  - name: localstack
    version: 0.6.16
    repository: https://localstack.github.io/helm-charts

Wrapper chart values.yaml

subdomain: foobar
environment: example

localstack:
  extraEnvVars:
    - name: SERVICES
      value: s3
  envFrom:
    - configMapRef:
        name: localstack-env-vars

localstack-env-vars-configmap.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: localstack-env-vars
data:
  LOCALSTACK_HOST: "{{ .Values.subdomain }}.{{ .Values.environment }}.internal"

Changes

The PR adds support for providing environment variables via ConfigMaps and/or Secrets via envFrom.

[alexrashed]

@breynolds-parachute, thanks a lot for your contribution!
Well described, useful enhancement, clean implementation! 💯
I also quickly sanity-checked the changes and it works perfectly fine!