Update of powheg scripts so to be able to use both LHAPDF 5 and 6

Example output LHE file of the updates is here:/afs/cern.ch/work/s/syu/public/LHE/cmsgrid_final.lhe
The updates include the following:

1) The example input cards now use new CT10 PDFs 11000 (instead of 10800).
2) create_powheg_tarball.sh checks if LHAPDF6 is used. 
   If LHAPDF6 is used, replace the nominal PDF set as 260000 and set LHAPDF environment variable 
 in runcmsgrid.sh
3) runcmsgrid.sh includes only 52 CT10 PDF weights variation for LHAPDF5.
 For LHAPDF6, weights variations include 100 NNPDF3.0 errors, 52+1 CT10 PDF, 16 CT10 alphas variations, central value of MSTW2008 NLO 68 CL, NNPDF 2.3 central value and two alphas variations.

I think you can just copy the logic from the amc@nlo scripts.

First LHAPDF_DATA_PATH is used to set the location, assuming that it's set correctly.  Then if the tools/available/lhapdf6.xml file is available it's used to overrwrite the location (and then LHAPDF_DATA_PATH is reset from the config script)