Declare variables "before" using them.

I thought there was no ordering at all in Puppet, but now I see:

Error: Could not retrieve catalog from remote server: Error 400 on SERVER: union(): Requires 2 arrays at /etc/puppet/environments/production/modules/tails/manifests/reprepro/snapshots/time_based.pp:86 on node apt.lizard

... so let's try this.