Generate passwords on first run

@samaradona @regilero now, defined vms/hosts have their root/sysadmin
password generated just in time, no more manual generation steps here.

This also means that our secret store becomes obsolete.

The near future way of having a box password is to go for mastersalt(mc_pillar.get_passwords) api.
Merge branch 'stable' of https://github.com/makinacorpus/makina-states into stable