ZEPPELIN-3328. Add plotting test for LivyInterpreter

### What is this PR for?
Add plotting for LivyInterpreter (Python & R)

### What type of PR is it?
[Improvement]

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-3328

### How should this be tested?
* CI pass

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Jeff Zhang <zjffdu@apache.org>

Closes apache#2866 from zjffdu/ZEPPELIN-3328 and squashes the following commits:

fd394e3 [Jeff Zhang] ZEPPELIN-3328. Add plotting test for LivyInterpreter

livy/src/test/java/org/apache/zeppelin/livy/LivyInterpreterIT.java

@@ -750,6 +750,26 @@ public void testSharedInterpreter() throws InterpreterException {
         assertEquals(1, result.message().size());
         assertTrue(result.message().get(0).getData().contains("col_1 col_2\n1 hello    20"));
       }
+
+      // test plotting of python
+      result = pysparkInterpreter.interpret(
+          "import matplotlib.pyplot as plt\n" +
+          "plt.switch_backend('agg')\n" +
+          "data=[1,2,3,4]\n" +
+          "plt.figure()\n" +
+          "plt.plot(data)\n" +
+          "%matplot plt", context);
+      assertEquals(InterpreterResult.Code.SUCCESS, result.code());
+      assertEquals(1, result.message().size());
+      assertEquals(InterpreterResult.Type.IMG, result.message().get(0).getType());
+
+      // test plotting of R
+      result = sparkRInterpreter.interpret(
+          "hist(mtcars$mpg)", context);
+      assertEquals(InterpreterResult.Code.SUCCESS, result.code());
+      assertEquals(1, result.message().size());
+      assertEquals(InterpreterResult.Type.IMG, result.message().get(0).getType());
+
     } finally {
       sparkInterpreter.close();
       sqlInterpreter.close();