run: Don't overwrite monitors when using --monitor

This patch changes the way default monitor is handled and uses
the usual way (cpu, images...). This makes it possible for ./run
to filter type instead of rewrite the list of monitors.

Previous solution didn't work for tests with multiple monitors.

**I have no strong opinion, whether this should be applied. Take it only as RFC.** (I'm fine with modifying `cfg/tests` to change the monitor for multi-monitors cases)