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 doesn't. While the ROS_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 quick fix to os_detect.py and perhaps some upstream logic to detect Centos as Rhel from the rosdep side.