ELK role

ELK is the cute marketing acronym for Elasticsearch, Logstash and
Kibana. This stack collects, augments and stores log events and provides
a GUI for searching the store.

This role also:
* configures MediaWiki to send log events to Logstash using Monolog
* configures rsyslogd to send HHVM events to logstash
* configures rsyslogd to send apache2 events to logstash

Change-Id: Ide8fb0fc0ca54d33f3af0c3adf7d4c412266ee50