Streams with same title are allowed

It is possible to create streams with the same title and description. While that is ok'ish on a entity model level, it creates problems when using the title field to distinguish between Streams.

## Expected Behavior

A streams' name should be unique so as to work as an identifier

## Current Behavior

Multiple streams with the same name can be created.
## Possible Solution

## Steps to Reproduce (for bugs)

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## Context

I use Graylog to provide Logging for my Kubernetes Cluster. I use the Graylog API to create one stream for each namespace present in my cluster. Next I want to start the Stream, which I can do with the 'stream_id' returned from the POST call. As my integration service is meant to be stateless, I do not save the stream_id/title combination.
Now a namespace gets deleted in Kubernetes and I want to also delete the stream associated with that namespace. To delete a stream, I need the stream_id. To get the stream_id, I need to query all streams and find the one I mean to delete. The only way to identify that stream seems to be the Title, so I get the first Stream with a matching title and delete it.

However since it is possible to create as many Streams with same title and description as I want, I can run into a situation where it is not possible to identify the stream I mean with certainty.

The same applies for Role creation for Stream. It is even visible in the UI:
**Multiple Streams Created:**

**How to distinguish between the Streams?:**

## Your Environment

* Graylog Version: 2.3.2-2
* Elasticsearch Version: 5.6.0
* MongoDB Version: 3.4.10
* Operating System: CoreOS ContainerLinux / Kubernetes (runs in Container)
* Browser version: Firefox 58