call install_package with -N

currently sometimes packages are removed from the isntallation list by
install_packages (e.g. linux-image-generic for Ubuntu 14.04). The
check of the package name using AptPkg is not working corret in some situations.

bin/fai-mirror

@@ -266,7 +266,7 @@ apt-get $qflag $aptoptions update >/dev/null
 set-classes
 [ $add -eq 1 ] && add_base_packages
 echo "Downloading packages for classes:" $classes
-FAI=$FAI_CONFIGDIR install_packages -d $vflag
+FAI=$FAI_CONFIGDIR install_packages -N -d $vflag
 umount_dirs
 trap "" EXIT ERR
 [ $add -eq 0 ] && delete_base_packages