slapos node: allow configure different pid files for each command

We only had a global "pidfile" configuration, but it's more conveniant to be able to configure different pidfile for each command (software, instance, report)

The supported config file would look like:

[slapos]
pidfile_software = /path/to/pid
pidfile_instance = /path/to/pid
pidfile_report = /path/to/pid