Adds Support for CentOS and Scientific Linux 7

CentOS Project has adopted a new rule for versioning numbers. The major number
matches the RHEL major number, but the minor number is generated from the
release date. For example, CentOS 7.0.1406.

Uses $::operatingsystemmajrelease instead of $::operatingsystemrelease for
avoiding issue like "Comparison of String with 7 failed" for CentOS 7.

Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>