Fix Debian 7 build failure: Make apt-get non-interactive to workaround interactive openssl update prompt

A recent openssl update in Debian 7 brings up an interactive prompt which hangs indefinitely, causing `veewee vbox build` to timeout (presumably `libssl-dev` https://github.com/jedi4ever/veewee/blob/master/templates/Debian-7.4.0-amd64-netboot/base.sh#L4)
![screen shot 2014-04-09 at 18 23 56](https://cloud.githubusercontent.com/assets/1644105/2660233/205edd18-c01d-11e3-853c-573a9e0917a1.png)
I've checked Debian-7.0-i386-netboot and Debian-7.4.0-amd64-netboot, the problem occurs in both.

This PR sets an environment variable which should force the default action to be taken without prompting.