Plugin Sanitation after rpm-based upgrade

- Version: 5.2.2
- Operating System: Centos 7.3
- Steps to Reproduce:

1. Install logstash via rpm: `rpm -i ./logstash-5.2.1.rpm`
2. Install a plugin: `/usr/share/logstash/bin/logstash-plugin install logstash-filter-dissect`
3. List plugins to see if it's there: `/usr/share/logstash/bin/logstash-plugin list |grep dissect`
4. Upgrade logstash via rpm (or yum): `rpm -U ./logstash-5.2.2.rpm`
5. List plugins to see if it's still there: `/usr/share/logstash/bin/logstash-plugin list |grep dissect`
a. It will not be listed.
b. `/usr/share/logstash/bin/logstash-plugin remove logstash-filter-dissect` says it is not installed
6. Check filesystem for plugin: `find /usr/share/logstash/ -name '*dissect*'`
a. results:
```
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/lib/jruby-dissect-library_jars.rb
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/lib/logstash/filters/dissect.rb
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/logstash-filter-dissect.gemspec
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/spec/filters/dissect_spec.rb
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/vendor/jars/org/logstash/dissect
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/vendor/jars/org/logstash/dissect/jruby-dissect-library
/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-filter-dissect-1.0.8/vendor/jars/org/logstash/dissect/jruby-dissect-library/1.0.8/jruby-dissect-library-1.0.8.jar
/usr/share/logstash/vendor/bundle/jruby/1.9/specifications/logstash-filter-dissect-1.0.8.gemspec
/usr/share/logstash/vendor/bundle/jruby/1.9/cache/logstash-filter-dissect-1.0.8.gem
```

Obviously, after upgrade, added-on plugins need to be re-installed, but holding onto the gems post-upgrade can get messy especially if there are new versions of those plugins. Then you'd have both sets of gems living on the filesystem.

A clean filesystem is a happy filesystem. :)