Change mechanism for specifying GPU ids

Using the -mdparam mechanism is inconvenient for when gmxtest.pl
needs to over-ride the number of MPI ranks for those tests that
do not run with more than (say) 1 rank.

Change-Id: I06c024d79fab1e88a8d41324629b34f09729c01a
Merge "Change mechanism for specifying GPU ids" into 5.0.0