Mass update: License identifier for all the plugins

Its seems that the license format we are currently using is using a **nonstandard** identifier

```
WARNING:  license value 'Apache License (2.0)' is invalid.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
Did you mean 'Apache-2.0'?
```

reported in https://github.com/logstash-plugins/logstash-output-s3/pull/99
