Make this configuration automatic

Assuming this configuration is intended for the use during the
development time, we can have Maven auto-insert the right value.

pom.xml

@@ -28,6 +28,9 @@
         <artifactId>maven-hpi-plugin</artifactId>
         <configuration>
           <defaultPort>8081</defaultPort>
+          <systemProperties>
+            <INSIGHT_UPLOAD_URL>http://localhost:8080</INSIGHT_UPLOAD_URL>
+          </systemProperties>
         </configuration>
       </plugin>
     </plugins>

src/main/java/io/jenkins/plugins/launchable/Ingester.java

@@ -24,6 +24,10 @@
 public class Ingester extends GlobalConfiguration {
     private Secret apiKey;
 
+    public Ingester() {
+        load();
+    }
+
     public Secret getApiKey() {
         return apiKey;
     }
@@ -54,7 +58,7 @@ public boolean configure(StaplerRequest req, JSONObject json) throws FormExcepti
             // attempted to use JDK HttpRequest, but gave up due to the lack of multipart support
             // TODO: how do I obtain a properly configured HttpClient for the proxy setting in Jenkins?
             try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
-                String endpoint = System.getenv("INSIGHT_UPLOAD_URL") ;
+                String endpoint = System.getProperty("INSIGHT_UPLOAD_URL") ;
 
                 if (endpoint==null) {
                     endpoint = DEFAULT_UPLOAD_URL;