Fix to detect the major release version for redhat/centos 7

[francis826]

In Redhat/CentOS 7, lsbmajdistrelease was not being set due to a minimal install not having all the required dependencies for lsbmajdistrelease and falling back to the default $os_rel = 6. This caused the puppet install to fail due to the wrong yum repo being set. As you can see below the differences in the facter output:

[vagrant@localhost ~]$ facter kernel lsbmajdistrelease
kernel => Linux
lsbmajdistrelease => nil

compared to:

[vagrant@localhost ~]$ facter kernel operatingsystemmajrelease
kernel => Linux
operatingsystemmajrelease => 7

Puppet version: 3.7
CentOS version: CentOS Linux release 7.0.1406 (Core)

After these changes were made, I tested on CentOS 5.10, 6.5, and 7.0

This may be related to issue #445

I am fairly new to Puppet and apologize for any guidelines that weren't followed while submitting this pull request.

[3flex]

Thanks for fix! Makes perfect sense.