Give operator permission to manager alertmanagers

Add a missing resource to the operator's cluster role allowing it to correctly
manager alertmanagers.

manifests/prometheus-operator/prometheus-operator-cluster-role.yaml

@@ -21,6 +21,7 @@ rules:
   - alertmanagers
   - prometheuses
   - prometheuses/finalizers
+  - alertmanagers/finalizers
   - servicemonitors
   verbs:
   - "*"