Fix ServiceMonitor selector

[joekrill]

The selector for the ServiceMonitor currently will not match the underlying service:

selector:
    matchLabels:
      app.kubernetes.io/instance: {{ $chart }}
      prometheus: unknown

1. app.kubernetes.io/instance: {{ $chart }} will only work if the release is named mail, because $chart is hard-coded to that value here. But that's not always the case.
2. prometheus: unknown will cause this to never match because this label isn't applied to the underlying service (unless it is explicitly set in the values provided the helm chart).

Fixes #162

[bokysan]

Thanks for the commit, merging!