Add Packer templates for Debian 7.4

* Add Debian 7.4 amd64 and i386 Packer templates
* Use same preseed file for all Debian 7 releases
* Fix Debian 7.2 and 7.3 default mirror URLs to point to the archive

Btw, I don't think there is much point in keeping the old point releases (7.2 and 7.3).

Also the json/host/etc. name could be just "7.4". Even though the CD URL includes the third digit (probably only for compatibility for the build tools), the official version number is "7.4", i.e. 4th update to Debian 7.

Happy to add commits for those if you agree. =)