DocumentDB support

## Expected Behavior
Graylog should work with AWS DocumentDB which is very similar to mongodb.

## Current Behavior
Graylog does not work with DocumentDB because it uses a function "capped" which is not supported by DocumentDB.

## Possible Solution
Maybe this function is not really needed and the same feature could be implemented in a way which does not need this function.

## Steps to Reproduce (for bugs)
```
2019-04-16T12:08:18.846Z INFO [connection] Opened connection [connectionId{localValue:4}] to docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017
2019-04-16T12:08:18.865Z WARN [GeoIpResolverEngine] GeoIP database file does not exist: /etc/graylog/server/GeoLite2-City.mmdb
2019-04-16T12:08:18.909Z INFO [connection] Opened connection [connectionId{localValue:5}] to docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017
2019-04-16T12:08:19.003Z INFO [connection] Opened connection [connectionId{localValue:6}] to docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017
2019-04-16T12:08:19.012Z INFO [connection] Opened connection [connectionId{localValue:8}] to docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017
2019-04-16T12:08:19.031Z INFO [connection] Opened connection [connectionId{localValue:7}] to docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017
2019-04-16T12:08:19.700Z ERROR [CmdLineTool] Guice error (more detail on log level debug): Error injecting constructor, com.mongodb.MongoCommandException: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }
2019-04-16T12:08:19.701Z ERROR [CmdLineTool] Guice error (more detail on log level debug): Error injecting constructor, com.mongodb.MongoCommandException: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }
2019-04-16T12:08:19.701Z ERROR [CmdLineTool] Guice error (more detail on log level debug): Error injecting constructor, com.mongodb.MongoCommandException: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }
2019-04-16T12:08:19.701Z ERROR [CmdLineTool] Guice error (more detail on log level debug): Error injecting constructor, com.mongodb.MongoCommandException: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }
2019-04-16T12:08:19.701Z ERROR [CmdLineTool] Guice error (more detail on log level debug): Error injecting constructor, com.mongodb.MongoCommandException: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }
2019-04-16T12:08:19.701Z ERROR [CmdLineTool] Guice error (more detail on log level debug): Error injecting constructor, com.mongodb.MongoCommandException: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }
2019-04-16T12:08:19.702Z ERROR [Server]

################################################################################

ERROR: Unable to connect to MongoDB. Is it running and the configuration correct?
Details: Command failed with error 303: 'Feature not supported: capped:true' on server docdb-2019-04-16-09-37-19.cbtciege4xon.eu-central-1.docdb.amazonaws.com:27017. The full response is { "ok" : 0.0, "errmsg" : "Feature not supported: capped:true", "code" : 303 }

```

## Context
This feature would allow users to run Graylog in AWS Fargate and use AWS DocumentDB and AWS Elasticsearch for the heavy lifting.

## Your Environment

* Graylog Version: 3.x
* Elasticsearch Version: na
* MongoDB Version: AWS DocumentDB