slap/standalone: let standalone's supervisord control instance supervisord

Instead of letting slapos commands start supervisord in daemon mode, run it
as a program from standalone supervisord daemon.

The $INSTANCE/etc/supervisord.conf will be created by `slapos node instance`
the first time user runs it, until this, the service will restart in a loop,
complaining that config file is not found.

Also review stop to expose timeout argument and use a longer timeout by
default (and cleanup some unused imports).