Installer: [FIX] look for Debian distro in given directory
Installer: [FIX] look for Debian distro in given directory

get_distro did not take the directory given as parameter into account when
looking for a Debian distribution. Instead it looked into / and thus found
that the Installer root image is not a Debian. Fixed.

tapper/Tapper-Installer,tapper/Tapper-Installer