Use container agents for faster test startup (#511)

Easier to administer container agents than full virual machines.

Also removes the setting of the jenkins version in the configuration
to null, since it is already set to null by default.

Jenkinsfile

@@ -1,7 +1,10 @@
 // While this isn't a plugin, it is much simpler to reuse the pipeline code for CI
 // allowing easy windows / linux testing and producing incrementals
 // the only feature that buildPlugin has that relates to plugins is allowing you to test against multiple jenkins versions
-buildPlugin(configurations: [
-        [ platform: "linux", jdk: "11", jenkins: null ],
-        [ platform: "windows", jdk: "11", jenkins: null ]
+buildPlugin(
+    // Container agents start faster and are easier to administer
+    useContainerAgent: true,
+    configurations: [
+        [ platform: "linux", jdk: "11" ],
+        [ platform: "windows", jdk: "11" ]
     ])