rgmanager: move nfsclient.sh cache files where they belong
    
    by using a static path to /tmp, the operation can be used to trigger
    a local DoS by a normal user.
    
    Move the cache files to /var/cache/cluster that's owned by root and
    doesn't allow normal users to play with it.
    
    Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>