Remove graylog.conf copy for Graylog 6.0

The repository currently uses a cusomized copy of the default `graylog.conf` file in `config/graylog.conf` plus an additional patch for the Graylog Enterprise image. (`patches/graylog-server.conf.patch`)

Maintaining a copy of the default config is a maintenance problem and can lead to shipping an outdated configuration.

For Graylog 6.0, we want to remove and replace the customized config file by setting default environment variables in the entry-point script. It must be possible for users to override these values!

We need to set working defaults for the following settings:

- `node_id_file`
- `bin_dir`
- `data_dir`
- `plugin_dir`
- `http_bind_address`

Users must configure the following settings:

- `mongodb_uri`
- `elasticsearch_hosts`

The entry-point script should validate that all required settings are configured.