Multiple logstash backend relations should cause an error

[afreiberger]

Currently, if I relate filebeat:logstash to graylog:beats and filebeat:logstash to logstash:beat, the charm allows for this configuration and configures all graylog unit and logstash unit IPs in the list of logstash hosts. Unfortunately, this means only some portion of my logs end up on each backend logging platform due to the nature of the loadbalance: true configuration.

I think it would be useful to block status if multiple backends are related, We should only be able to relate one single application to either logstash or elasticsearch backends, as the filebeat spec only allows for a single output to be defined in filebeat.yml.