The node.name Logstash config variable is currently hard coded
to inventory_hostname in the elastic_logstash role. This commit
exposes the option to modify this, which allows Logstash to be
identified under a more readable name in e.g. Kibana. The default
behaviour is unchanged.
Change-Id: Ic3b21d54c9759117550cc85bade3dc7900d0f502
2a1a47457b