Fix race condition in puppet_statsd by deferring config file check

Deferring the configuration file loading to the actual report callback ensures
that (a) it runs at the end of the Puppet run, and thus after
/etc/puppet/statsd.yaml would have been populated, and (b) that it doesn't run
at all if the statsd reporter isn't configured.

Change-Id: I4cea6f54ebac80e7ab580beb9d9134808b808012