Jessie desktops

just opening this for review, comments welcome

still 3 major issues left that I'm aware of:

* d-i asks non-preseeded question (rt#3049)
* udev names network interface eth1 (rt#3050)
* puppet-trigger doesn't work on jessie (rt#3052)
You would think not, but I just tested again... if I move that line to the bottom of `desktop::init`, I get:

    Nov 10 23:56:38 hozer-53 puppet-agent[6376]: Could not retrieve catalog from remote server: Error 400 on SERVER: Duplicate declaration: Class[Desktop::Xsession] is already declared; cannot redeclare at /opt/puppet/env/jessie/modules/desktop/manifests/init.pp:25 on node hozer-53.ocf.berkeley.edu

There's probably something else weird happening (possibly wrt `require` vs `include` or similar).