small fixes

fixed a bug with filter beeing always applied, even if empty.
added new subcommand -l to only show the hosts
changed filter parsing.

we now only pass the filter array to the backend.
renamed --show-hosts to --list-hosts.
d'oh.
fixed a bug where CPX::Distexec.load_nodes has been called twice.
backend config option does now have filter attribute.

can be an array or string.