Add .deb-based build to Jenkins scripts

I agree - I had just not set up the preseeding/debconf stuff to handle the 'debs. I will attack this after my holidays unless anarcat wants to jump in there and mess about with the fabric commands.

I think for actual testing of releases, we should always use debs. Just that the testing phase has to come after we've basically rolled out the release, which is awkward (but perhaps not so bad, we can always reroll the debs I presume?)

We have a 6.x-1.x branch test which uses the manual method, so that can stay as is, and is in fact the more 'common' build as it's automated everytime a commit is pushed.