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

```



/reviewed-on !29