[CI] Refactor Jenkins CI pipeline + migrate all Linux tests to Jenkins (#4401)

* All Linux tests are now in Jenkins CI
* Tests are now de-coupled from builds. We can now build XGBoost with one version of CUDA/JDK and test it with another version of CUDA/JDK
* Builds (compilation) are significantly faster because 1) They use C5 instances with faster CPU cores; and 2) build environment setup is cached using Docker containers

jvm-packages/xgboost4j-tester/src/test/java/ml/dmlc/xgboost4j/tester/AppTest.java

@@ -0,0 +1,20 @@
+package ml.dmlc.xgboost4j.tester;
+
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+/**
+ * Unit test for simple App.
+ */
+public class AppTest 
+{
+    /**
+     * Rigorous Test :-)
+     */
+    @Test
+    public void shouldAnswerWithTrue()
+    {
+        assertTrue( true );
+    }
+}