[alertmanager] Conditional volume mounting for configmap reload

[hemstreet]

What this PR does / why we need it

Optional volume mount when using config map reloader. Without this, the deployment will fail if you have config.enabled set to false since it can't find the volume.

This error would show up in the event logs when adding the config reloader

create Pod alertmanager-2 in StatefulSet alertmanager failed error: Pod "alertmanager-2" is invalid: spec.containers[0].volumeMounts[0].name: Not found: "config"

With this change, you can then specify the volume mount if you are using a secret / etc using configmapReload.extraVolumeMounts and the service will deploy as expected.

cc: @monotek @naseemkullah

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])