Configtest doesn't complain about missing read permissions but logs error on startup

- Version: 2.3.2 (but seems to apply to other versions as well)
- Operating System: Ubuntu 14.04.3
- Config File (if you have sensitive info, please remove it): -
- Sample Data: -
- Steps to Reproduce:
- Create config in `/etc/logstash/conf.d`
- Remove permissions on that config
- Run `service logstash configtest` -> `Configuration OK`
- Start LS and it will complain about missing permissions

E.g:

```
# ls -la /etc/logstash/conf.d/
total 12
drwxrwxr-x 2 root root 4096 May 11 10:52 .
drwxrwxr-x 3 root root 4096 May 11 10:32 ..
---------- 1 root root 769 May 11 10:52 test.conf
# service logstash configtest
Configuration OK
# service logstash start
logstash started.
# cat /var/log/logstash/logstash.log
{:timestamp=>"2016-05-11T11:13:35.509000+0000", :message=>"Permission denied - /etc/logstash/conf.d/test.conf", :level=>:error}
```

Would it be possible to check the permissions with `configtest` and throw an error rather than when LS is being started?