Move include of role::cache::varnish::kafka::statsv into prodution realm conditional

This keeps varnishkafka from being set up in labs and beta, where there
are no kafka clusters.

"Error: Could not retrieve catalog from remote server: Error 400 on SERVER:
keys():  Requires hash to work with at /etc/puppet/manifests/role/cache.pp:388
on node  i-0000007f.eqiad.wmflabs"

Bug: T76799
Change-Id: Ie5c6a274876b170bf4f7a65d4feeaf17f64610b7