Add constraint selector to pod template

PodTopologySpread plugin will only count the existing pod when that
pod's label matches with `constraint.Selector`, which means all pods
could be scheduled to one topology zone when the constraint does not
have any selector defined.

Signed-off-by: Dave Chen <dave.chen@arm.com>

test/integration/scheduler_perf/config/pod-with-preferred-topology-spreading.yaml

@@ -2,11 +2,16 @@ apiVersion: v1
 kind: Pod
 metadata:
   generateName: spreading-pod-
+  labels:
+    color: blue
 spec:
   topologySpreadConstraints:
     - maxSkew: 5
       topologyKey: topology.kubernetes.io/zone
       whenUnsatisfiable: ScheduleAnyway
+      labelSelector:
+        matchLabels:
+          color: blue
   containers:
   - image: k8s.gcr.io/pause:3.2
     name: pause

test/integration/scheduler_perf/config/pod-with-topology-spreading.yaml

@@ -2,11 +2,16 @@ apiVersion: v1
 kind: Pod
 metadata:
   generateName: spreading-pod-
+  labels:
+    color: blue
 spec:
   topologySpreadConstraints:
     - maxSkew: 5
       topologyKey: topology.kubernetes.io/zone
       whenUnsatisfiable: DoNotSchedule
+      labelSelector:
+        matchLabels:
+          color: blue
   containers:
   - image: k8s.gcr.io/pause:3.2
     name: pause