[ZEPPELIN-1845] InterpreterFactory.createOrGetInterpreterList fails when interpreter mode is "perUser": "scoped"

[prabhjyotsingh]

What is this PR for?

When trying to add a paragraph, whose default interpreter is running in mode "perUser": "scoped" fails with following exception

ERROR [2016-12-22 11:52:18,784] ({qtp112466394-59} NotebookServer.java[onMessage]:318) - Can't handle message
java.lang.NullPointerException
	at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:936)
	at java.util.concurrent.ConcurrentHashMap.containsKey(ConcurrentHashMap.java:964)
	at org.apache.zeppelin.interpreter.InterpreterFactory.createOrGetInterpreterList(InterpreterFactory.java:1219)
	at org.apache.zeppelin.interpreter.InterpreterFactory.getInterpreter(InterpreterFactory.java:1333)
	at org.apache.zeppelin.notebook.Paragraph.isValidInterpreter(Paragraph.java:598)
	at org.apache.zeppelin.notebook.Paragraph.getMagic(Paragraph.java:587)
	at org.apache.zeppelin.notebook.Note.setParagraphMagic(Note.java:506)
	at org.apache.zeppelin.notebook.Note.insertParagraph(Note.java:316)
	at org.apache.zeppelin.socket.NotebookServer.insertParagraph(NotebookServer.java:1330)
	at org.apache.zeppelin.socket.NotebookServer.onMessage(NotebookServer.java:241)
	at org.apache.zeppelin.socket.NotebookSocket.onWebSocketText(NotebookSocket.java:59)
	at org.eclipse.jetty.websocket.common.events.JettyListenerEventDriver.onTextMessage(JettyListenerEventDriver.java:128)
	at org.eclipse.jetty.websocket.common.message.SimpleTextMessage.messageComplete(SimpleTextMessage.java:69)
	at org.eclipse.jetty.websocket.common.events.AbstractEventDriver.appendMessage(AbstractEventDriver.java:65)
	at org.eclipse.jetty.websocket.common.events.JettyListenerEventDriver.onTextFrame(JettyListenerEventDriver.java:122)
	at org.eclipse.jetty.websocket.common.events.AbstractEventDriver.incomingFrame(AbstractEventDriver.java:161)
	at org.eclipse.jetty.websocket.common.WebSocketSession.incomingFrame(WebSocketSession.java:309)
	at org.eclipse.jetty.websocket.common.extensions.ExtensionStack.incomingFrame(ExtensionStack.java:214)
	at org.eclipse.jetty.websocket.common.Parser.notifyFrame(Parser.java:220)
	at org.eclipse.jetty.websocket.common.Parser.parse(Parser.java:258)
	at org.eclipse.jetty.websocket.common.io.AbstractWebSocketConnection.readParse(AbstractWebSocketConnection.java:632)
	at org.eclipse.jetty.websocket.common.io.AbstractWebSocketConnection.onFillable(AbstractWebSocketConnection.java:480)
	at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
	at java.lang.Thread.run(Thread.java:745)

What type of PR is it?

[Bug Fix]

What is the Jira issue?

• ZEPPELIN-1845

How should this be tested?

Outline the steps to test the PR here.

Screenshots (if appropriate)

Before:

After:

Questions:

• Does the licenses files need update? N/A
• Is there breaking changes for older versions? N/A
• Does this needs documentation? N/A

[prabhjyotsingh]

CI Green, ready for review.

[r-kamath]

LGTM

[prabhjyotsingh]

Merging this if no more discussion.