Update the method string handling for NFS and URL installs.
    
    yum doesn't understand nfs: and nfsiso: URLs, so we've been mangling them
    early in the anaconda file.  Now with stage2=, we need to do this later
    because in the NFS case, we won't have the source repo mounted.  Delay the
    mangling until we're doing repo setup, which is also where the NFS source
    will get mounted.