Allow multiple instances

Allow multiple varnishkafka instances per node by changing the ::varnishkafka
class to a custom resource type -- ::varnishkafa::instance.

Each instance is managed via an upstart config /etc/init/varnishkafka-${name}.conf
and a config file /etc/varnishkafka/${name}.conf.

Tested.

Change-Id: I205f92c72e3da23771ce2223b6eb8ee48ac95c5b