Adding Centos to the list of detectable OSes.

This fix adds a CentOS class to the detecting scheme. Since Rhel and Fedora, exist in the list already, I'm not sure why CentOS hasn't. While OS_OVERRIDE environment variable can be set to allow tools like rosdep to work on CentOS, there's no actual reason to not have CentosOS support if all it is, is a few lines in python.
Fixed this to use the base OsDetector, Rhel isn't necessary.

Sure. I wasn't sure about thats since, as shown, it's not quite in order, but I've changed that too.