rework repo list generation for iso building

* include BUILD_TYPE=ALPHA option for alpha repos
* move ovirt repos to point to node-base rather than core ovirt
* include ovirt repos for CENTOS/EL6 builds

Change-Id: I658c65041429fcade1a56ae0b5d627c3e843efe6
Signed-off-by: Mike Burns <mburns@redhat.com>