Add option to list descriptions without details

The `list` command gets the `--quick` option to list all system
descriptions without listing any details. This is much quicker.
It is useful when looking up names of descriptions without being
interested in any details.
Merge branch 'quick_list'

Pull request #395

We might want to make the code consistent in terms of default values
for command line options, but I would do this with a separare pull
request as it also affects other options.
Merge branch 'master' of github.com:SUSE/machinery