Use same preseed file for all Debian 7 releases

The files were and most probably always should be identical.
Add packer templates for Debian 7.4
Update Debian 7.2 and 7.3 default mirror URLs

They are now archived after Debian 7.4 release.
Use builder specific output directories also for Debian 7.3 and 7.4
Remove old Debian 7 update releases
Rename Debian 7.4.0 to Debian 7.4

The official version is "7.4" even though the download URL uses the
extra digit.
Merge pull request #164 from tmatilai/debian-7.4

Add Packer templates for Debian 7.4