[alertmanager] default to 1 replica, multinode gate uses 3

Change-Id: Ifb1420f8dcf7237349a79f1f97aea5e547bafeab

prometheus-alertmanager/values.yaml

@@ -52,7 +52,7 @@ pod:
       alertmanager:
       init_container: null
   replicas:
-    alertmanager: 3
+    alertmanager: 1
   lifecycle:
     upgrades:
       revision_history: 3

tools/deployment/multinode/060-alertmanager.sh

@@ -21,7 +21,8 @@ make prometheus-alertmanager
 
 #NOTE: Deploy command
 helm upgrade --install alertmanager ./prometheus-alertmanager \
-    --namespace=osh-infra
+    --namespace=osh-infra \
+    --set pod.replicas.alertmanager=3
 
 #NOTE: Wait for deploy
 ./tools/deployment/common/wait-for-pods.sh osh-infra