move creation of service account to role 'common'

* maybe a better name is needed
* reason is that we do not necessarily want to apply role 'base' in
  jails
add new role -- supervisor

* add supervisor package
* config it
* enable and start supervisor daemon
* the role has a parameter -- services -- a list of supervisor services
  to be created: see roles/supervisor/templates/service.conf.j2 for
  parameter list (probably should go to a overall doc of sort)
wip