[ZEPPELIN-4892] Delete plugin dir in clean phase
[ZEPPELIN-4892] Delete plugin dir in clean phase

### What is this PR for?
This PR will delete the content of the plugin folders. Unfortunately the main folder is still present after a `mvn clean`

### What type of PR is it?
- Bug Fix

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-4892

### How should this be tested?
* **Travis-CI:** https://travis-ci.org/github/Reamer/zeppelin/builds/699667326

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

Author: Philipp Dallig <philipp.dallig@gmail.com>

Closes #3808 from Reamer/clean_plugin_dir and squashes the following commits:

a0dab671c [Philipp Dallig] Delete plugin dir in clean phase

prabhjyotsingh/zeppelin,prabhjyotsingh/incubator-zeppelin,zjffdu/zeppelin,prabhjyotsingh/zeppelin,jongyoul/incubator-zeppelin,joroKr21/incubator-zeppelin,hammertank/zeppelin,joroKr21/incubator-zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,hammertank/zeppelin,apache/incubator-zeppelin,fogbeam/zeppelin_mirror,VipinRathor/zeppelin,hammertank/zeppelin,hammertank/zeppelin,joroKr21/incubator-zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,apache/incubator-zeppelin,VipinRathor/zeppelin,jongyoul/incubator-zeppelin,joroKr21/incubator-zeppelin,apache/zeppelin,jongyoul/zeppelin,jongyoul/zeppelin,jongyoul/zeppelin,VipinRathor/zeppelin,fogbeam/zeppelin_mirror,hammertank/zeppelin,prabhjyotsingh/zeppelin,prabhjyotsingh/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,apache/zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,jongyoul/incubator-zeppelin,joroKr21/incubator-zeppelin,apache/incubator-zeppelin,fogbeam/zeppelin_mirror,zjffdu/zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,jongyoul/zeppelin,VipinRathor/zeppelin,zjffdu/zeppelin,zjffdu/zeppelin,prabhjyotsingh/incubator-zeppelin,apache/incubator-zeppelin,jongyoul/zeppelin,zjffdu/zeppelin,apache/incubator-zeppelin,apache/zeppelin,apache/zeppelin,jongyoul/incubator-zeppelin,jongyoul/zeppelin,hammertank/zeppelin,apache/zeppelin,apache/incubator-zeppelin,fogbeam/zeppelin_mirror,apache/zeppelin,prabhjyotsingh/incubator-zeppelin,jongyoul/zeppelin,hammertank/zeppelin,VipinRathor/zeppelin,jongyoul/incubator-zeppelin,prabhjyotsingh/zeppelin,apache/zeppelin,prabhjyotsingh/incubator-zeppelin,fogbeam/zeppelin_mirror,fogbeam/zeppelin_mirror,jongyoul/incubator-zeppelin,prabhjyotsingh/zeppelin,fogbeam/zeppelin_mirror,joroKr21/incubator-zeppelin