SLC Carbon support

Hello, (first time I do a pull request, sorry if I made any mistake

I have made minimal changes to the module so it works with: Scientific Linux CERN SLC release 6.3 (Carbon). Which is basically a clone of redhat6.

I also added a default statement in manifests/redhat.pp so if $lsbdistcodename is not defined, it fails. I forgot to define it, and the error puppet gave me was far from useful (an error in the user creation).