Add a missing switch to support for CentOS 7.0

In base role, only CODENAME_MAJOR=6 is actually use, this change enable
CentOS 7.0 to be built without the following error:
"Cannot find a valid baseurl for repo: base/$releasever/x86_64"
Merge pull request #110 from TristanCacqueray/master

Add a missing switch to support for Centos 7.0