Added new "osfinger" grain

This grain is a unique identifier for a given OS and OS version that can
also be used as the key in a dictionary. For example a fingerprint for
CentOS 5.9 would be "CentOS-5", a fingerprint for Precise would be
"Ubuntu-12.04".
Merge pull request #6561 from whiteinge/grain-osfinger

Added new "osfinger" grain