Add ping_time parameter to target.config and reboot

Created by: mdigiorgio

When rebooting, the reboot function repeatedly pings the target. However, rebooting might require some seconds and in order to avoid pinging the machine before it actually reboots, we can set the ping_time option in target.config to tell the function how long to wait before pinging.

Also, a comment has been added in target.config to specify what the file represents and to highlight that commented parameters are optional.