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.
Does this need Rhel here? Can it not use the base OsDetector?

And can you put this between Arch and Cygwin to sort of keep alphabetical order. It's not all in alphabetical order but we can keep new classes in order.