execute clean command without asking confirmation

we need python-progressbar >= 2.3 to work with is.
We previously ship inside it because debian doesn't have 2.3 package available.
now we have built a debian package, we can remove code and add a dependency

old command line option are still present but no more in config file
this allow to override easily on command line config parameters

--no-sync as the same beneficts

this commit also make cachify more exception safe

if no filter given, list in all online repository
Also remove list -S, can be done by is -s ''

also use Repository.is_repository_name to validatage repository name

This commmit make a big cleaning inside IS binary, moving all code insides functions
and removing unecessary use of variables

also optimize tarball.getnames

dangerous options are last one

prepare and uprepare chroot functions now track non existant files
preparing are now done more carrefully about tricks

repo command can now purge cache of database

python-argparse (1.1) shipped with debian squeeze is bugged and doesn't work with installsystems needs
python-argparse 1.2.1 (last upstream version) was backported into squeeze and we need it.

Thanks to Antoine "Naps" Millet for its precisous help

list -S only list images inside searchable repositories