OS informations are broken using facter in CentOS 7.1 : os variable is not populated as it should be, and so operatingsystemmajrelease and operatingsystemrelease are not created at all.
I think this comes from the /etc/redhat-release file in centos 7.1
In Centos 7.0, /etc/redhat-release is a symbolic link to /etc/centos-release which contains "CentOS Linux release 7.0.1406 (Core)
In Centos 7.1, /etc/redhat-release is a regular file which contains "Derived from Red Hat Enterprise Linux 7.1 (Source)"
Modifying /etc/redhat-release to be a symlink to /etc/centos-release solves the problem.