tcpircbot: subscribe to (rather than require) config

Requiring the config means the bot doesn't get restarted if the configuration
changes, as did in fact happen with change Ib33d3cc66: the bot was not
restarted by the Puppet run that made the configuration change.

Change-Id: Icd89287f06c289c34ea71940e22447999404245b
Merge "tcpircbot: subscribe to (rather than require) config" into production