Change the way how precompiled tar ball is copied to /afs/cern.ch/cms/generators/www/

Previous version automatically copied precompiled powheg tarball to the directory 
 /afs/cern.ch/cms/generators/www/<scram_arch_version>/<beamenergy>TeV/powheg.

To avoid overwriting the existing tar balls, we modify the script. This version put precompiled tar ball 
in a local working directory. Then, the tar ball must be copied to the afs generator directory by the MC 
contacts.