RFE: add a way to exlucde modules from generated images

[rpigott]

We can currently include extra modules with the "modules: " option in the config, but Booster could use a config option to exclude certain modules as well.

I would suggest a "modules-skip" or "modules-exclude" option for this purpose.

[anatol]

A new "exclude" operator is added and it can be combined with modules elements. For example if one wants to exclude module foo then it will look like modules: -foo or exclude all modules is modules: -*.

Please try master and let me know if you see any issues.