Add support for CentOS Stream 8 repos
    
    This patch makes that CentOS Stream 8 systems use repositories under
    8-stream in cloudsig instead of 8 ones.
    
    It also moves to use centos-release-advanced-virtualization rpm to
    configure Advanced Virtualization repo.
    
    Finally, it does some cleanup on excludes for centos8 which were from CentOS
    7 but are not longer required.