switch/cray: set PMI_CRAY_NO_SMP_ENV

This fixes the setting of PMI_CRAY_NO_SMP_ENV for v15.08.
The previous commit (25b03b8c)
worked fine with v14.11, but changes were required to function
with v15.08 and commit 54e53857,
which gives user control over packing on a job-by-job basis in
order to over-ride system with packing configuration.
See bugs 819 and 1585