AptBackend: use force-confdef to avoid terminal input

we don't have stdin and can't select anything during dpkg configure so
we have to use the default config options.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
Merge pull request #860 from rbbratta/dpkg_force_confdef

AptBackend: use force-confdef to avoid terminal input