Add tasksel and taskel-data to exclude option in FAI_DEBOOTSTRAP_OPTS.

Grml doesn't use tasksel{,-data} and dropping tasksel saves us
1,868kB disk space. Therefore lets get rid of it by default
when debootstrapping the chroot.

Thanks: Tong Sun <suntong@cpan.org>
Signed-off-by: Michael Prokop <mika@grml.org>