Support FluxHelmRelease values format with more than one image

[squaremo]

At present fluxd knows how to interpret and update FluxHelmRelease manifests that use the format

values:
  image: repo/image:tag

which admits only pretty simple charts, since it only lets you change one image. An improvement would be

values:
  foo:
    image: repo/foo:tag
  bar:
    image: repo/bar:tag

where each image is under a label, which can be treated as the "container" for that image (most likely that's what the templates will use the values for).