fixed OS detection & ensure python bz2 support

I think the OS detection can be improved using lsb_release instead of checking /proc/version.

I've just found a real case (an ubuntu custom cloud image) with a wrong OS detection (debian instead of ubuntu).

```
# cat /proc/version 
Linux version 2.6.32-28-pve (root@lola) (gcc version 4.4.7 (Debian 4.4.7-2) ) #1 SMP Tue Mar 18 15:21:47 CET 2014
```

```
# lsb_release -s -d
Ubuntu 12.04.4 LTS
```