stack/monitor: run monitor-globalstate once from buildout

monitor-globalstate (which is supposed to create monitor configuration file
accessed by HTTP as .../public//monitor.global.json) runs by cron every 2
minutes. This crontab also uses randomsleep to add a delay - but there's a
promise waiting for this monitor configuration file to be available, so the
instanciation had to wait for this and if this was sometimes causing problems
in software release tests because promise did not succeed fast enough.

To prevent this pattern where slapos node instance has to wait for a crontab
to be executed, we run that script a first time from buildout.