makeTest: Pass nodes attribute

This allows doing things like:

  nix-build nixos/tests/misc.nix -A nodes.machine.config.system.build.initialRamdisk
Really fix NixOS/nixos#287
Mount unionfs-fuse filesystems using the normal fileSystems option
Remove Subversion from the installation CD
Shut up a warning in stage 2
sec: update to version 2.7.6
firefox-bin: update to version 31.0
nixos: tmp on tmpfs option

/tmp cleaning is done by systemd rather than stage-2-init
enableEmergencyMode moved from systemd to seperate module
new option to mount tmp on tmpfs
new option to enable additional units shipped with systemd
add source-han-sans-japanese, source-han-sans-korean, source-han-sans-simplified-chinese, source-han-sans-traditional-chinese: An open source Pan-CJK typeface
source-han-sans: fixed package name
postgresql: Update to 9.0.18, 9.1.14, 9.2.9, 9.3.5