jenkins: Force iSCSI volume on specific nodes

Instead of routing iSCSI traffic from the Kubernetes network, through
the firewall, to the storage network, nodes now have a second network
adapter connected to directly to the storage network.  The nodes with
such an adapter are labelled `network.du5t1n.me/storage`, so we can pin
the Jenkins PersistentVolume to them via a node affinity rule.

jenkins/iscsi.yaml

@@ -47,4 +47,11 @@ spec:
     fsType: ext4
     secretRef:
       name: iscsi-chap
-
+  nodeAffinity:
+    required:
+      nodeSelectorTerms:
+      - matchExpressions:
+        - key: network.du5t1n.me/storage
+          operator: In
+          values:
+          - 'true'