slaprunner: provide pidfile in config file for slapos node

Set default pidfiles for slapos node software and slapos node instance
command so that when they are executed they use by default the same
pidfile that are used by slaprunner GUI ( the green "play" button ).

This prevents potential problems with running slapos twice and fixs the
problem that default pidfile ( /opt/slapos/slapgrid-sr.pid ) is not writable.