Support typical chart image format in FluxHelmRelease automation

[oliviabarrick]

The current FluxHelmRelease automation looks for image: name:tag in the values.

Most Helm charts use a different format:

image:
  repository: name
  tag: tag

It looks like it would be pretty trivial to support both formats, just have different handling for maps than strings: https://github.com/weaveworks/flux/blob/2d58c32ae195fda77450177e7d16bbc554c26975/cluster/kubernetes/resource/fluxhelmrelease.go#L63

Example:

$ helm create hello
$ cd hello
$ head values.yaml
# Default values for hello.
# This is a YAML-formatted file.
# Declare variables to be passed into your templates.

replicaCount: 1

image:
  repository: nginx
  tag: stable
  pullPolicy: IfNotPresent
$

Supporting this format would make helm-operator work with most charts out of the box.