[alertmanager] Add an option in values file to allow use existed Volume Claim or existed Persistent Volume

[duchuyvp]

Is your feature request related to a problem ?

This is a feature request, I'm working on bare-metal k8s and use nfs to provisioning PV to PVC. But it's nice if I can manually create PV before and alertmanager chart allow me to use that PV.

Describe the solution you'd like.

there is an option in values file persistence.volumeName (like prometheus helm chart) or persistence.existingClaim (like grafana helm chart)

I know how statefulset works so maybe also values file could support an deployment type as using Deployment kind.

Describe alternatives you've considered.

NONE

Additional context.

And last but not least, as I mentioned above, I can manually create PV but it's good if people can add PV template in .Values.extraObjects so that PV belongs to helm.