Logstash: Add support for logging irc !log messages

Add logstash::input::irc and filter configuration for storing !log
messages seen in subscribed irc channels. Also add role::logstash::beta
which can be applied in labs to enable logstash::input::irc.

Change-Id: I409fec919bce3d1340c44212f58f89104f2ff203
Merge "Logstash: Add support for logging irc !log messages" into production