I’m not sure if there is an ARM version available… The ISO you are using is i586 / x64, which is not going to work on RPi 2’s CPU.
Afaik, there is no CentOS for the RPi either, but there is http://pidora.ca/, which is probably the closest thing there is currently.
That said, I’m not sure putting Rockstor on a RPi is going to all that useful? (fun, but if you just want to fileshare some USB devices there are lighter options)