added support for unstable debian releases

We use a custom build of Debian. I updated the build scripts to support this situation.
The `lsb_release -c -s` command returns the string "n/a" on this release.

Maybe this is useful for someone else and AppScale can also support this scenario.

```
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux testing/unstable
Release:	testing/unstable
Codename:	n/a

```