`elasticsearch_hosts` doesn't allow characters as part of parameter

## Expected Behavior

In an environment where I've standardized on using `_`'s as part of hostnames in my environment, I should be able to provide a hostname with a `_` as part of the array of values in `elasticsearch_hosts`

## Current Behavior

When providing a hostname with a `_` as part of the name, I receive the error:

```
2021-01-18 16:22:18,143 ERROR: org.graylog2.bootstrap.CmdLineTool - Invalid configuration

com.github.joschi.jadconfig.ValidationException: Parameter elasticsearch_hosts must not contain URIs without host or scheme. (found [http://graylog_es01:9200, http://graylog_es02:9200, http://graylog_es03:9200])
```

While is is fixed by changing the `_` to a `-`, this is not easily remedied in large environments where naming conventions are standardized and would require effort to change.
## Possible Solution

Create a patch to allow `_`s to be valid characters in hostnames in the `elasticsearch_hosts` parameter

## Steps to Reproduce (for bugs)

1. Create a dockerized Graylog deployment where the elasticsearch service/container name contains a `_`
2. Add the elasticsearch host with `_` to the `GRAYLOG_ELASTICSEARCH_HOSTS` env var
3. Bring the deployment up
4. Observe the error