WIP: stack/monitor: Add promise runner script
This script reuses the promise launcher from slapos.core to launch the instance promises using the python executable installed by the SR.
EDIT: Note that the slapos.core egg is already installed by stack/monitor anyway since it's a dependency of slapos.toolbox and is needed for the plugin promises.
This MR exists in conjunction with slapos.core!329 (merged).