`release.yaml` outputted from `make validate` should merge with existing contents

[aiyengar2]

Instead of starting with an empty ReleaseOptions{} here:

charts-build-scripts/pkg/validate/validate.go

Line 46 in bc4c3da

releaseYaml := options.ReleaseOptions{}

We should be loading the release.yaml from the existing file (if it exists), since all of our walk functions will not track existing items in the release.yaml that will be dumped anyways:

charts-build-scripts/pkg/validate/validate.go

Lines 97 to 103 in bc4c3da

	if releaseOptions.Contains(chart.Metadata.Name, chart.Metadata.Version) {
	// Chart is tracked in release.yaml
	return nil
	}
	// Chart exists in local and is not tracked by release.yaml
	logrus.Infof("%s/%s is untracked", chart.Metadata.Name, chart.Metadata.Version)
	response.UntrackedInRelease = response.UntrackedInRelease.Append(chart.Metadata.Name, chart.Metadata.Version)