mpi/mvapich: Don't set MPIRUN_PROCESSES by default

The MPIRUN_PROCESSES variable set by the mpi/mvapich plugin probably
is not needed for most if not all recent versions of mvapich.
This environment variable also negatively affects job scalability
since its length is proportional to the number of tasks in a job.
In fact, for very large jobs, the increased environment size can
lead to failures in execve(2).

Since MPIRUN_PROCESSES *might* be required in some older versions of
mvapich, this patch disables the setting of that variable completely
only if SLURM_NEED_MVAPICH_MPIRUN_PROCESSES is not set in the job's
environment. (Thus, by default MPIRUN_PROCESSES is disabled, but
the old behavior may be restored by setting the environment variable
above)