Add held nodes to singlestat for nodepool dashboard

Currently we need to ssh into nodepool.o.o to see how many nodes are
held.  Add them to grafana too, so others can see.

Change-Id: I316a4ad9060538fab027630c3898713c3eabe435
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

grafana/nodepool.yaml

@@ -24,7 +24,7 @@ dashboard:
           type: singlestat
           valueName: current
         - title: Ready
-          span: 3
+          span: 2
           sparkline:
             full: true
             show: true
@@ -33,7 +33,7 @@ dashboard:
           type: singlestat
           valueName: current
         - title: In Use
-          span: 3
+          span: 2
           sparkline:
             full: true
             show: true
@@ -42,7 +42,7 @@ dashboard:
           type: singlestat
           valueName: current
         - title: Deleting
-          span: 3
+          span: 2
           sparkline:
             full: true
             show: true
@@ -50,6 +50,15 @@ dashboard:
               - target: sumSeries(stats.gauges.nodepool.provider.*.nodes.delete)
           type: singlestat
           valueName: current
+        - title: Hold
+          span: 3
+          sparkline:
+            full: true
+            show: true
+          targets:
+              - target: sumSeries(stats.gauges.nodepool.provider.*.nodes.hold)
+          type: singlestat
+          valueName: current
     - title: Zuul Launchers
       showTitle: true
       height: 320px