[chronograf] Support configurable common labels

[lovetoburnswhen]

Adds the extraLabels property to values.yaml which allows users to provide custom labels to be added to generated resources.

This feature would be tremendously useful for environments that require certain labels to be set - the only alternative would be to fork the chart internally.

---

❯ helm template ./charts/chronograf -s templates/deployment.yaml --set 'extraLabels.foo=bar'                             
---
# Source: chronograf/templates/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: release-name-chronograf
  labels:
    app: release-name-chronograf
    chart: "chronograf-1.2.7"
    release: "release-name"
    heritage: "Helm"
    helm.sh/chart: chronograf-1.2.7
    app.kubernetes.io/name: chronograf
    app.kubernetes.io/instance: release-name
    app.kubernetes.io/version: "1.9.4"
    app.kubernetes.io/managed-by: Helm
    foo: bar
spec:
...

---

• [ ] CHANGELOG.md updated None found
• [x] Rebased/mergable
• [x] Tests pass
• [x] Sign CLA (if not already signed)