Implement --quiet

[glance-]

This program is quite verbose in it's regular running form, so this implements a --quiet flag which lowers the log-level to WARNING, getting rid of all the normal status messages, and will only say something if something unexpected happens.

[xoxys]

While custodian seems to be dead, I've create a fork and slightly reworked it. Maybe someone else find it useful. https://github.com/xoxys/docker-tidy