Per suggestion by @bangerth, allow mpirun to execute as root if two
envars are set to specific values
Per conversation with @jsquyres, name the envars OMPI_ALLOW_RUN_AS_ROOT
and OMPI_ALLOW_RUN_AS_ROOT_CONFIRM
Fixes#4451
Signed-off-by: Ralph Castain <rhc@open-mpi.org>
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>