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>
Merge branch 'AddsSupportCentOS7' of github.com:strider/puppetlabs-mysql into AddsSupportCentOS7