Servermanager.cmd will also return 1003 (already installed) which is a valid return value
Shift servermanagercmd_returns into a proper private variable...
If you put array in variable, Ruby does weird things. So don't.
Other servermanagercmd return codes: 3010 for "success, reboot", and 1001 "failure, reboot"
Add a note about 1003 causing a reboot + CHANGELOG