Graylog web_endpoint_uri is not configurable via charm

Graylog has several settings used to configure access for the API and Website. The primary three are web_listen_uri, rest_transport_uri, and web_endpoint_uri.

The charm is currently missing the ability to configure web_endpoint_uri - this makes it impossible to access the Graylog website from an external IP address when using a reverse proxy - such has haproxy. The proxy forwards web traffic to the web_listen_uri, but Graylog returns the rest_transport_uri as the default value for web_endpoint_uri - which is typically configured as an internal IP address used for cluster communications.