VerifyJSONConfigs verify every elements in Data. (#9302)

Add error message in the velero install CLI output if VerifyJSONConfigs fail.
Only allow one element in node-agent-configmap's Data.

Signed-off-by: Xun Jiang <xun.jiang@broadcom.com>

pkg/nodeagent/node_agent_test.go

@@ -249,6 +249,7 @@ func TestGetConfigs(t *testing.T) {
 	cmWithValidData := builder.ForConfigMap("fake-ns", "node-agent-config").Data("fake-key", "{\"loadConcurrency\":{\"globalConfig\": 5}}").Result()
 	cmWithPriorityClass := builder.ForConfigMap("fake-ns", "node-agent-config").Data("fake-key", "{\"priorityClassName\": \"high-priority\"}").Result()
 	cmWithPriorityClassAndOther := builder.ForConfigMap("fake-ns", "node-agent-config").Data("fake-key", "{\"priorityClassName\": \"low-priority\", \"loadConcurrency\":{\"globalConfig\": 3}}").Result()
+	cmWithMultipleKeysInData := builder.ForConfigMap("fake-ns", "node-agent-config").Data("fake-key-1", "{}", "fake-key-2", "{}").Result()
 
 	tests := []struct {
 		name          string
@@ -331,6 +332,14 @@ func TestGetConfigs(t *testing.T) {
 				},
 			},
 		},
+		{
+			name:      "ConfigMap's Data has more than one key",
+			namespace: "fake-ns",
+			kubeClientObj: []runtime.Object{
+				cmWithMultipleKeysInData,
+			},
+			expectErr: "more than one keys are found in ConfigMap node-agent-config's data. only expect one",
+		},
 	}
 
 	for _, test := range tests {