Use label to identify pods belonging to the Frappe deployment

[PatrickFranken]

As the Kubernetes-based deployment of ERPNext requires RWX volumes, it's rather difficult to deploy on a cluster where the infrastructure provider does not offer such volumes. NFS is a workaround but also difficult to deploy right in a production environment. Due to this, IMHO it's a valid option to place all ERPNext pods on one single node which allows so share a usual RWO volume.
This scheduling can be achived using

nginx: 
  nodeSelector:
    system/ais-deployment: default
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        - labelSelector:
            matchLabels:
              app.kubernetes.io/app: frappe
          topologyKey: kubernetes.io/hostname

(The same has to be applied for all deployments, nginx is used as an example here)

This MR adds a common label to all pods, as this is required for this kind of affinity.

[PatrickFranken]

@revant any chance to review and merge this? Not sure about the failed check, looks like it's not related to these changes. Let me know if you need any more information.

[revant]

https://github.com/frappe/helm/blob/main/CONTRIBUTING.md#run-pre-commit

Run this and commit the changes. That'll fix lint failures.

[PatrickFranken]

@revant sorry, should have read that before. Branch is now up-to-date.