Fix building nodes panel for zuul launchers

We will never have manager nodes that are set to building, because
zuul-launcher does not work that way.  Switch back to target for
building.

Change-Id: I20f841473987b5099b7869ae69a4ce3cca696e71
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

grafana/nodepool.yaml

@@ -81,7 +81,7 @@ dashboard:
         - title: Building Nodes
           span: 3
           targets:
-            - target: aliasByNode(stats.gauges.nodepool.manager.*.nodes.building, 4)
+            - target: aliasByNode(stats.gauges.nodepool.target.*.nodes.building, 4)
           type: graph
         - title: Ready Nodes
           span: 3