make AMQP SSL connection trust every certificate optional

## Expected Behavior
Like with any other Input that can be used with SSL it should be possible to tell if the certificate must be valid or not.

## Current Behavior
any certificate can be used - see log message

```
2018-01-05T16:21:44.120+01:00 INFO [AmqpConsumer] Enabling TLS for AMQP input [Syslog AMQP/5a4f97bb221ae10d142537f4].
2018-01-05T16:21:44.120+01:00 WARN [TrustEverythingTrustManager] This trust manager trusts every certificate, effectively disabling peer verification. This is convenient for local development but prone to man-in-the-middle attacks. Please see http://www.rabbitmq.com/ssl.html#validating-cerficates to learn more about peer certificate validation.
2018-01-05T16:21:44.128+01:00 INFO [InputStateListener] Input [Syslog AMQP/5a4f97bb221ae10d142537f4] is now STARTING
```

## Your Environment

* Graylog Version: 2.4