Add a matchLabels selector to stateful sets as well

[aduffeck]

This is required to work around
helm/charts#7726, without it we see errors like

Error: UPGRADE FAILED: StatefulSet.apps "nats" is invalid: spec.template.metadata.labels: Invalid value: map[string]string{"app.kubernetes.io/managed-by":"Tiller", "app.kubernetes.io/name":"my-nats", "app.kubernetes.io/version":"", "helm.sh/chart":"my-nats-10", "skiff-role-name":"nats", "app.kubernetes.io/component":"nats", "app.kubernetes.io/instance":"my-nats"}: `selector` does not match template `labels`

during helm upgrades.

[cfdreddbot]

Hey aduffeck!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.